Okay, so here's a little more detail. Sorry about that. I put the whole boat back together so no pics, unfortunately. I'm not taking it apart again until I know how to fix it -- it was a pain in the arrse to remove the panel inside the trunk, between the ballast and the fuel tank (where the shower is mounted).
Okay, there are a red and a blue line (obviously hot and cold) that run from beneath the engine, under the floorboards, above the gas tank, and connect to the shower unit in the trunk. Nowhere visible is any sort of a bilge pump or similar (as was expected) in line on either of these hoses. With a seperate line that connects to the red hot water line, there is a large black cylinder unit, about 6" tall by 3" diameter. It looks similar to a bilge, but upon taking it apart, I realized there is no impeller or anything like that to pump water. Hmmm... I don't know what this unit is, but my guess is it is some kind of a heater, since it appears to only touch the red (hot) water line.
So that was how far I got when I tried to fix last time.
My problem is that I don't have any water pressure on the cold or hot settings -- It's pretty much dead on both. I get a hot water trickle, but no pressure. This leads me to believe that there's got to be another pump somewhere along the line that I don't know about.
Or...could that weird cylinder contraption be a pump in disguise.
So... do any y'all know: is the heat in the shower created by the engine block or a seperate heater? If it's a separate heater, it is probably that weird cylinder thing I tried to take apart. But if it's heated by the engine block, then I'd guess that weird cylinder thing is my problem...

Who knows. I sure as hell don't. Too bad I'm mad at my local shop right now. They got a new service guy that has no sense of customer service whatsoever. I'm getting desperate and debating taking the boat into a Mastercraft Dealer (Cope & McPhetres).