To answer some of your questions:
Yes, ship combat is all about ship boarding and your units fighting it out, with the victor capturing the ship - Rise and Fall style.
But, not to say that something stops player packing the ship with catapults and fire-arrow archers for more traditional ship-to-ship combat, or ship-to-land assaults.
Yes, we did gave up on regular units swimming as a special ability, well not completely, but it`s unlikely we’ll give this ability to all of our units.
And on these screens these are dead bodies “swimming”, they remain in water, as well as some props after the ship is sunk.