As old as the tolet is, if you've never replaced the air valve in the pump, that would cause the symptoms you describe...the air valve is on the front on the pump housing...a little square "nut" that looks like it's molded into the housing, but in fact threads into. It's not a part in a rebuild kit. Give Vic Willman at Raritan a call to discuss and order: 800-352-5630 x 6

If that's not it, the problem has to be downstream of the toilet...'Cuz what you're describing sounds like back pressure. That could be caused by blocked or partially blocked tank vent (most common locations are the thru-hull and the connection to the tank)...or a buildup of sea water minerals in the head discharge line that's reduced the diameter severely....a clog...possibly a kinked hose. You'll have to check each till you find it.