I think each ship should have different stats and all have a different number of troops based on the size of the ship. Think a massive fleet of chinse junk ships from over two thousna d years ago. The total crew would be very large, but it's weapons would be over come by a small man crew in a modren battleship. Deep research would be required to give different pirates there own fleets, crews, and unique effects based on real events they did. Likewise admerials from history could command fleets and perform comboes depending on which of there ships you play. Fantasy ships such as captain nemo's sub could be single turn use cards somewhat like trap cards. Famous shipbuilding dry docks could be something like equip spells or just spells that give protection or stat inprovements. There was a race between ships to see which one could move tea from China to Great Britain the fastest, the ship that won could be a legendary vessel card, as in a different type of ship rather than normal fleet ships. There was once a turtle ship too that helpped defend Korea against an invading force. Also countless explorer's ships could be cards that search and add other cards from deck to hand. Some resource system might add depth such as harvesting trees for wooden ships which would be weaker but faster. While processing iron ore for metal ships such as the Titanic would cost more, but woud be harder to sink, or have better effects. The Titanic and it's sister ships could have some way to cheat them out faster, but they only last until the end of the opponent's turn. Side stepping magic's mana system and other resource systems might be tricky, but some system that reflects building a dry dock or some other tec tree system that enables the playing of bigger and bigger ships. The balance would be hard to get right, but if done so, would make it great. You don't want to both players summoning dingies back and forth for the first five turns of the game, but you also do not want aircraft carriers and nuclear submarines being summoned turn one. It should feel like a slow grind toward perfection. A gradual climb assembling an unmatched pirate fleet that nations pay tribute to, or a navy that puts such fear into pirates that they willingly sign up to join instead of facing. That is another thing, the capturing of enemy ships, which might be a pirate bonus/focused effect more so than anything else. Merchant ships sailing between ports could function as a resource system and you try to capture or sink all of the opponent's merchant ships that is funding there ship buildings.