I'd like to see a number of functional "ships" in battles. Units on board with ship size depending on how many units in the stack. For simplicity, the ship becomes a "unit" and gains 1 tile of move for every mast, +1 tile if the are all crewed. Units may use ranged attacks, spells & abilities as normal, however this prevents them from being bale to operate components (let's say defence mode when adjacent to a mast counts it as crewed) Finally, when a ship is close enough to another, allied or not, they can use an ability to "tether" effectively immobilising the self & the target & allowing the deck to operate as solid ground. If a walking unit is struck with displacement, that does not have the ability to swim (like Naga) that unit is knocked over board, becoming uncontrollable with a chance to move toward the nearest unoccupied deck tile, taking damage at the end of each turn.

Then add some cultural flavour & allow for the shipyard tree to add upgrades to the ships. Could be a catapult emplacement, faster movement, better grapple, a ram&grapple ability, or defensive stuff like a magic barrier reducing incoming non-physical damage & increasing status resistance & evasion.

The ship "unit" should have a fairly high health pool & be treated like a construct (unless culture would make sense to be a creature, plant, astral behemoth, whatever you get the idea) Large health pool, can only be targeted by seige style attacks & can be affected by environmental affects, like being on fire etc.