If you're just taking the pistol out on day trips and not living on the boat or doing long cruises, most any quality pistol will do. Sig, Berretta, Glock or a quality 1911 would all work even if they got dunked. The key is to clean them when you get home, especially if you boat on saltwater. Also decent Smith & Wesson revolvers or Rugers.
What do you want it for, gators, snakes, sharks, or bad guys?
My grandfather would take out an old Mauser rifle in the ocean, or if he was running in rivers he took the rifle or a single shot shotgun.
Some of the guys down here in S.E. Georgia will keep a .44 revolver or a cheap rifle in the boat in case they see hogs on the river banks.