show us a pic of the head.
most have a switch that you can push up or down. Pushing UP will run both the water pump that rinses the bowl and the pump on the bottom of the toilet itself. Down will just activate the pump on the bottom of the toilet. Try another fuse and just use the Down button. IF the fuse blows again you have a problem with the toilet pump, not the rinse pump.
you can unbolt the toilet to get to the bottom pump. when I bought my boat it did the same thing as you describe. there ended up being something jammed in the bottom pump keeping it from spinning and it would pop the fuse. I unjammed it and all worked fine.
Here is a pic of my head. dont have a pic of the switch though.