Several possible causes
First, make sure the seacock is open (oh....duh!)
If it is, and if you have a strainer on the intake, make sure it isn't clogged...if so clean it out. If you don't have a strainer, the intake line--or even the holes in the rim of the bowl--may be clogged...and even with a strainer, the intake line could be clogged with sea water mineral deposits. Or...as Mike suggested, you may have an air leak in the intake hose.If you've never rebuilt the toilet, the pump shaft seals and piston o-ring could be so worn that the pump won't prime any more....worse yet, have worn the pump cylinder to the point where new ones won't help...or sand/debris may have scored the pump housing...which in either of those cases means you need a new pump assembly. So start checking at the seacock till you find the problem. If you can't find it, it's almost certain that the problem is in the pump itself. Since a rebuild kit costs about 40% of the cost of a new pump assembly, if the toilet is more than 5-7 years old and has never been rebuilt, consider replacing the pump assembly instead...because rebuilding it may not cure it.