By state order Marinas were not essential businesses and should have been closed. If it had been closed, the boat could not have loaded passengers, also in violation of the state covid order, and made the fateful trip.
Regardless of the "obvious" fault of the captain, an argument can be made that the marina shares fault by being open in violation of the order. If they are not brought to the table, any fault that falls on them can not be recovered. Also, with their lack of participation, the other parties will be working to place as much blame on them as possible.
It's not frivolous to bring everyone that could be even be partially responsible into the suit, because there is no second chance.