Thanks, I understand your concept better now.
However for this one:
But placing the card isn't required. If you don't want the card, don't place it.
I take it that you have limited space in your boat so if it's full before you start a fight, you'd be sure not to have any space left if you get new cards after the fight. So if placing new card isn't required, where would the new card go? assuming the boat is full.
I asked this thinking that your boat is the only container for your cards so if you haven't expanded your boat and your boat is full, you can't put new cards there at all. You can't go to shop to sell excess card to solve this.... or maybe, this option is given in boat setting/card placement section.