Steve is right...
The question is, SHOULD you replace the toilet or repair it? That depends on the make/model/age of the toilet...'cuz all marine toilets are NOT created equal! So...whatchagot?
If it turns out that replacing the toilet is the best option, that's easy...toilets are mounted using lag bolts, so no nuts in some inaccessible location to deal with. But you do need to know that the mounting bolt pattern for any new toilet is not likely to be the same as the pattern for the old toilet, so you'll prob'ly have at least a couple of new holes to drill and a couple to fill.
And btw...it would be a good idea to close all the seacocks in the whole sanitation system BEFORE removing any equipment or disonnecting any plumbing.
And now we prob'ly oughta talk about hoses and the holding tank...???
The good news is, you can do everything you need to do while the boat is in the water.