Several of the marine heads have parts availability and various service kits which may help you out. As Tom as asked above, the brand and model are important. It seems from your post that the head on your boat has not had much attention or service--they do last longer and operate better if they get some service. Either check the operators manual, or give us make and model and someone will have advice on what service it needs. Usually adding some head lube helps; vinegar run through the hoses decreases calcium buildup, replacing valves is inexpensive and very easy and makes a difference, etc.
Frank