The problem with using sealants, etc. in the fuel system (past the filters) is that anything which works free, is now free to get to your injectors and make an expensive problem. Even if the diaphragm is made of Unattainalon (similar to Unattainiam<G>) it will eventually wear, so I side with the chorus that says REBUILD
THE PUMP. New diaphragm, clean the seats and it will seat right, and not let you down.
If the diaphragm actually tears or pinholes from old age, it can also let fuel into your lube oil
since the rocker arm usually is open to the cam in the engine. And that, in turn, could cause all sorts of grief. If your engine oil
level never seems to go down but actually INcreases? that can be the cause. Worth fixing ASAP.
I'd be in favor of installing an auxiliary electric pump
, if for no other reason than most boaters use the starter in order to work the mechanical lift pump when they are bleeding the fuel lines. And using the starter for more than 30 seconds per ten minutes (will vary with the maker, etc.) *will* cause overheat damage and eventually kill the starter. Which will be more expensive than an electric