If it flushes into a holding tank, the first thing to check is the tank vent for a blockage, especially if the toilet is "burping" and backing up.
If discharge is just sluggish, low voltage to the toilet is the leading cause.
You're in salt water...sea water mineral buildup can reduce the toilet discharge hose to 1/2", which would definitely slow down discharge.
Depending on the age of the toilet, it may just be a worn out discharge impeller...or a failed pump wall (part in the pump that looks like a CD)...in which case I strongly advise putting a whole rebuild kit in it, not just the one or two parts that have already failed 'cuz the rest aren't far behind.
Check all those possibilities...and if it's not one of 'em, give Raritan tech support a call: 800-352-5630