Marine heads rely on Joker valves and Oneway valves to keep things where they belong.. so to speak.
Both of these Valves are rubber and relies on “sealing” against another rubber surface to seal. Sea water is not good for these and builds up on them, creating a seal problem.
A little olive oil from time to time and I mean a little, a capful once in a while, will help with this enormously.
First thing is that olive oil manages to dissolve salt deposits and the second one is it lubricates the rubber seals and helps them “seal”.