Plumbers non-return valve. Available in threaded metal, brass is the obvious economic choice, bronze is dearer, but also in PVC and ABS for gluing or screwing.
A ball inside, usually a floating ball, allows water
to flow out of the boat but not in. Cheap
to buy and therefore replace, pretty strong, and a fit and forget item that only needs attention/inspection at the annual refit
These will also help where water
is pumped in by holding water above them (heavy ball sinks against the seat stopping water flowing out of the in pipe) and ideal for basin or toilet sea water supply.
I never got my 2" brass seacocks to move at all, they were left open over a long previous ownership
and had seized solid. Plastic would have done the job just as well but would not have passed the insurance survey
because they might have been prone to being knocked off. Now you can swing on these things at two inch bore, take them apart (split body screws together) and a child can open/close them, even a lady. I'd prefer to go PVC or ABS for all boat plumbing