I honestly am still on the fence with this one. . . I agree with lostieloo that if we don't see a body, then they usually aren't dead. Plus, I don't think Jin would be stupid enough to stay on the boat knowing it is going to explode. Not to mention the fact that we don't see anyone on the deck when it explodes. BUT, if he did by chance get off the boat in time, where could he have gone?? I think his only chance for living would be if Faraday and the Zodiac picked him up. But if the Zodiac got sucked down with the island, Jin would just end up S-O-L. Unless he was smart enough to grab some type of floatation device before jumping. Then he would float along in the ocean for a few days, and then be S-O-L. Either way, it doesn't look promising for Jin, but I have high hopes!
Here's another theory on how Jin could have survived, if he was able to dive off the boat before the explosion and survive the jump and the blast concussion:
As has been said, the Island would be gone, there would be no place for him to swim.
Jin would presumably be stranded just like the O6 plus Des and Frank.
But...we know that Halliwax, on the tape, warned against putting non-organic material in the chamber, especially metal. Suppose only organic matter can transport (through time and/or space) with the island
It could be possible that Jin would transport with the Island while the gang in the chopper would not, even though closer to the Island, because they are encased in metal, the chopper itself. After all, we don't know how wide an area would transport with the Island. Perhaps Jin, alone and totally organic in the water, would fall within its influence. As for Daniel and the others in the zodiac, could go either way, but I'd bet they would go with the Island as well because of their nearness to it.
Sun may have used her control of Paik Industries to discover this possibility, but needs to manipulate Widmore into some action to make her plan work, hence her trip to London.
Hell, it would make for one very satisfying reunion in season 5 or 6, especially after her agony at apparently losing him in the explosion.