"It must have been too fast for him to regenerate."
As if. He's had the most terrible injuries before regeneration. Drowning or burning wouldn't have stopped him. He wouldn't even have needed to regenerate, probably. He could've run. If he'd just got back to the TARDIS, the medical bay, he'd've been right as rain.
Because that was just it, he wouldn't be right as rain, not at all. He'd've been about as healthy as a dying horse or as right as rain that went up. He'd still be dying, because no one heard him whisper his last words:
"Last chance to say it... Rose Tyler, I love you."