The heater is easy. You just have to take the time to fit the pieces before you drill holes or mount anything. I'd suggest that you install ball valves on both inlet and outlet lines so you can turn it off or work on it without losing the use of the boat.
I think you could get hot water for the shower from the heat exchanger on your closed cooling system without too much trouble. The water to the exchanger is pumped in so you should have enough pressure to get warm water out of it to shower. It won't be as hot as that from a fresh water system though. I have not installed a shower, but I'm putting in a second heater this winter.
I got mine from www.biggunsmarine.com
in Texas for good prices. The hot tubes are really the way to go.