I refuse to believe that Gangplank is dead

For the simple reason that there is no body.

Anyone who's ever read comic books or watched a movie knows that, when a character supposedly "dies" in an accident, but there is no body, they're generally still alive. And Gangplank didn't become the uncrowned king of Bilgewater by being easy to kill.

Furthermore, Riot's said, in their own official announcement, "We encourage all Gangplank fans to remain calm for a few days until we can fully assess the situation." If he was truly dead, I would expect a calm reassertion of the fact, not a "wait and see." Act IV, baby.

Besides, with no other named characters left in Bilgewater to contest MF, it'd be a pretty boring "Battle for Bilgewater."