Author's Notes:

The usual disclaimer - I own nothing of the Sonic world and inhabitants, nor those of the Pirates of the Caribbean world.

And now, welcome to what I do believe is at the time I write this, the one and only Sonic the Hedgehog/Pirates of the Caribbean crossover!
Of course, if people write more, I'll have to change that, but in time, in time.

This is mostly set, as the name suggests on Mobius. As for Jack, I've decided he's got slightly lost looking for the fountain of youth, because his compass has picked up he wants a ship, which in turn manages to lead him to Mobius.
Barbossa and the Black Pearl might show up later. I haven't decided yet. You're looking at the start of the idea I had, and it taking form as I write it.

And now, enough of my rambling, and on with the start of the story.

Sonic couldn't believe what he was hearing.
"Treasure hunting? Out at sea? Did someone replace Eggman with a fake while I wasn't looking?"
"At least it's keeping him out of trouble," Tails told him.
"Yeah, but if it's only him out there, he's gonna have it all to himself."
"You're not thinking of-"
"Getting it for ourselves? Why not? Can't go letting him think he's not gonna have to share with us."
"Sonic, we don't even have a ship."
"You build one, and I'll help then."
"You're terrible with tools."
"Didn't say anything about tools."
"Better ask the others if they want to help out too, then. Rouge will be interested, no doubt."
"Already on it," he replied, speeding out and leaving Tails to design their ship. Tails was just as interested, he could tell. He just didn't want to disturb Eggman enough to set him on them again.
He couldn't resist though – when did they ever let him get away with anything? And stealing treasure, well... better them than him.
Knuckles, as usual, was dozing at Angel Island. He really needed to get out and about more. He was almost as bad as Shadow for being unsociable, but at least he was a reliable friend.
"Hey, Knuckle-head! Up for another adventure?"
"What're you up to this time?"
"Me? Up to something?"
"It's quiet, Eggman's not doing anything. So you're probably up to something."
"Eggman's hunting for treasure at sea."
"Let him. I prefer the peace we've got for a change."
"Aw, come on. Don't tell me you're not gonna help us go hunting too."
"What for? If you want to, go ahead."
"What if Eggman thinks we're stealing his treasure?"
Knuckles sighed. "Then you'll have to deal with him, won't you? I'm not interested, Sonic. I want a break from it all for a change."
"Suit yourself. We just won't cut you a share of whatever we find."
"Hey, wait up! You never mentioned that."
"Didn't think you were interested."
"If you'd told me that, I might have been."
"Go let your girlfriend know then."
"She's not my girlfriend."
"Whatever you say," he grinned back. "If you and Rouge want a share, you gotta help out."
"Oh, alright. Tails is building a ship, you say?"
"I'll be there."
Sonic left him, already heading on to Shadow's. Knuckles had been easy, but Shadow was going to be a challenge.
"What's in it for me?" was his first question.
"Same as everyone else, share of whatever we get."
"Besides that."
"What more do you need? It's not like we're gonna be doing anything else."
"You mean, nothing like maybe boarding Eggman's ship and taking his finds?"
"Eh, only if he gets in the way. Besides, he doesn't really need it."
"You're talking piracy, Sonic."
"No I'm not. I'm just suggesting we take it from him before he uses it for another plan for us to foil."
"Yeah, when you do that at sea, it's called piracy."
"He just has to keep out the way then."
"You'll make me captain then."
"Actually, Tails has first choice on that. It'll be his ship."
"First mate."
"You'll have to talk to the captain about that."
"So you're recruiting a crew for a ship, without knowing where you'll be on the crew, and not being able to promise anything, even any finds, because you don't know?"
"It's his ship, not mine. Besides, no doubt he'll have something lying around to find us the stuff."
Shadow said nothing.
"If you're not interested, just say so. 'course, I can't see why we'd need a gloomy fake hedgehog out at sea. I just thought I'd ask you out of courtesy."
"You don't think you need me? I'll show you."
Shadow left, pulling Sonic outside with him. Sonic knew Shadow wouldn't trust him leaving him in his own home.

Charmy came flying into the Chaotix office, slamming into Vector the way he always did when excited.
"You won't believe this, you won't believe it!"
Vector pulled himself up. "Won't believe what?"
"Tails is going hunting for treasure at sea, and wants a crew for his ship. We could get rich helping them!"
"There's no guarantee he'll find anything," Espio murmured from his corner, still not opening his eyes.
"Don't be a spoilsport – I heard Sonic say Eggman's looking as well, so there's gotta be something!"
"What's the catch?" Vector asked, putting his desk back in some kind of order.
"Oh, we got to help build the ship and be part of the crew, but everyone's getting a share of whatever gets found. Maybe we'll run into Eggman and get some of his too!"
"That's called piracy, Charmy," Espio interrupted. "It's wrong."
"So's Eggman always trying to take over the world, and if he doesn't have any treasure to spend, he can't do it."
"He's got a point, Espio. Least we can do is find out if Tails will let us on his ship."
Espio knew the signs. Once Vector had the idea of getting paid into his head, there was no getting it out again.
"Let's go," he told them shortly, leading them out.
Tails turned out to be ordering parts ready for use when they got there. He shushed them into silence, finishing the order, then turned to them.
"Captain Tails, what can I do for you?"
"Captain?" Vector asked.
"Well, it's going to be my ship. I assume that's why you're here, right?"
"Yeah, who told you?"
"I guessed. Knuckles and Rouge have already offered to come along, Shadow got persuaded by Sonic. I was going to call you, actually."
"We hear you're offering a slice of what's found to all who help?"
"It's only fair."
"What about the first mate?"
"You're in competition for that one. Sonic, Shadow and Rouge are already trying to get me to pick them."
"Count me in the running too, then."
"If I remember right," Espio broke in, "Sonic can't swim. What is he doing coming along?"
"It was his idea in the first place. He convinced me to build him a ship."
"Wouldn't that make him captain?"
"Not on my ship," Tails replied.
With Tails directing their efforts, organising everything, and checking over the work, their ship was finished in short order. Tails insisted on calling it the Treasure Hunter, but after a brief argument it was shortened to just the Hunter. It resembled an old sailing ship, except with some more modern technology on board.
Vector, much to Shadow's annoyance, had been awarded the position of first mate because he was a natural leader – his leadership of the Chaotix team proved that. Tails felt he could trust him with the job. Shadow had sulked over it, but admitted that it was better than having Sonic get the job.
After having lost out on the position and to everyone's surprise, Sonic commandeered the galley, turning out to be a fair cook, something none of them realised he could do.
Espio took the crow's nest, to keep watch. Tails had insisted it wasn't really necessary, but he'd done it anyway, and got Charmy to join him. Charmy would be able to fly down and let them know what he saw, since Espio almost never raised his voice for anything.
Knuckles and Rouge provided security, having made the addition of a brig, 'just in case we need it' and also took charge of the ship's armaments.

Jack looked around. There didn't seem to be anyone there.
He moored his little dinghy up at the docks, and looked over the strange looking ship. It looked perfectly normal, except it wasn't made of wood.
It'd make a perfect ship for reaching the Fountain of Youth before Barbossa did, he decided, climbing aboard.

"So, where to first?" Vector asked Tails, who'd found out a three-pointed hat to mark his position as captain.
"Open sea, of course. We'll be able to use some of the equipment I brought aboard to locate places worth looking at."
There was a clatter outside.
"Everyone is aboard, right?"
"Yeah, we're all here."
"I'll take a look," Knuckles told him, leaving.
They waited.
"Hey! What are you doing?" someone's voice cried out. "This is my ship!"
"I don't think so," Knuckles told the stranger. "You're not the captain."
"Ah, I beg to differ," the stranger replied. "For I am Captain Jack Sparrow."
"Never heard of you."
Tails and the others left the cabin and went on deck. Knuckles was holding an odd looking human against the mast.
Tails approached.
"I'm the captain," he said. "And this is my ship. So what are you doing aboard it?"
"Oh, nothing much. I was commandeering it."
"You couldn't sail this ship without me. I made it."
"Oh, a shipwright and a captain? That's a rare thing."
"Captain?" he grinned back, already having an idea of what Tails was going to say.
"Throw him overboard, will you? You can swim, I assume, Captain... Sparrow, wasn't it."
"Yes, Captain Jack Sparrow. I really don't think you want to be throwing me overboard."
"Why not?"
"You don't look like experienced sailors. I, on the other hand, am a well experienced sailor, and can be very useful to you."
"Prove it."
Sparrow pushed on of Knuckles' fists aside, and started rummaging in his various pockets. He pulled out a bottle of something, which he took a drink from, then continued.
A pistol turned up, which he looked at in mock confusion before stowing it away again, then he found a sword bucked to his waist. They looked at him in astonishment.
"Can't be too careful at sea, you know," he quipped, then continued searching himself.
He finally pulled out a small black box on a piece of string.
"This," he proclaimed grandly, "Is why you don't want to throw me overboard."
"It's a box."
"Much more. Inside this box is a compass that does not point north."
"You have a broken compass."
"No," he replied in a pained tone. "It works just fine, thank you. It points to the thing the owner wants most."
Tails considered that.
"Knuckles, if he doesn't hand it over, throw him overboard anyway."
"Hey, wait! You can't do that!" Sparrow protested.
"My ship. My rules. You hand it over if you want to stay on board. Oh, and you'll take orders from me, like the rest of the crew."
"I can't do that, I'm a captain."
"You're not the captain of my ship, Jack Sparrow."
"Captain," he insisted.
"Throw him off."
Sparrow jumped back, drawing the sword.
"Now, listen here. I need a ship. You have one, and a crew to boot. I'm looking for something, and I'm going to take this here ship to find it. Savvy?"
"You hand over the compass, and once we're finished, we'll send you on your way to finding your own treasure. Get it?"
"You'll hand me the ship?"
"A ship," Tails corrected. Jack didn't seem to notice the correction.
"I think we have an accord, then. I want the compass back, mind."
"Of course you'll get it back."
"Done, then." He handed the compass over in one hand, and shook Tails hand with the other. He noted it looked somewhat... unclean, and there was a distinctly unpleasant smell about him.
"You need a shower," Tails commented.
"Nothing a good rainstorm won't handle next time I'm in one. Are we going?"