A pic or clearer description would really help. Some boats have the cap and gasket above for rod holders. They really should not be required on a boat as all rod holders should drain into the cockpit. Think of the crud that can get on them when fishing.... The cup holders either drain into the cockpit or helm floor, or have a cap at the bottom (which kinda sucks as the water builds up in the cup holder) or they have a tube that drains the water away - usually to the bilge where it will be pumped out assuming the pump is triggered.