I noticed today that Wizards have released some rules on waterborne adventuring.
A quick look over them seems to indicate that the minotaur with their bonus action shoves on melee attacks and goring charge are a nice option for melee classes especially. Mariner I think is a good option for my game, Curse of the Black Pearls because getting a consistent swim speed is pretty difficult – but it does come at the cost of heavy armor (which does make sense). The new Rogue style looks especially good as adding your charisma to your initiative is pretty huge (especially if you take the alertness feat for another +5 or whatever it is). Finally the storm sorcerer seems to be pretty cool as well and rather thematic. I’ll be interested to see if any of my players take these options.