Many fishing boats (anyone remember fishing boats?) had "wet holds" to keep fish alive & well untill delivered to the fish dock.
Basic idea is to build an enclosed watertight hold then drill holes in the bottom to let water in and stop fish getting out. As long as sides of hold are above waterline, boat doesn't sink. When caught, fish are dumped in hold & swim around in the sea water swilling thro' the wet hold. Process continues until fish don't have room to swim any more.