When I built my system I got one of these pumps
http://www.princessauto.com/all-seas...e-utility-pump
then I bought a shower head from Home Depot with a 10' hose on it. I just cut the threaded part on the end of the shower head off and attached it to some more hose to reach the back of the boat.
Use a Y splitter like this
374330617_446.jpg
Where you see the water tap is where you would put the pump. Each leg of the splitter should go to hot or cold water source. I tapped off the block for hot water, and put a T into my water line in between the impeller and the water pump on the motor. You put your shower head on the other side of the pump. For the first try you will need access to that Y fitting. You need to adjust the levers on there to get your water temp set. Mix the hot and cold until its how you like it. Then you just need a switch for the pump. Turn it on when you want the shower to go, and off when you dont.
There may be a bit of syphoning of the water if you have the whole plumbing system mounted low. A loop that goes up high under the side of the boat will help negate this.
If you find that pump cheap enough you can build the whole system for under $100.
The thing to think of here is that a shower is GREAT for everyone who is getting in the water. We all loved it, and it helped us a TON with getting in and out of the water when it was so cold. The problem is that a heater would have been nice for when we were NOT wet yet, but still cold. We were driving the boat with a full winter coat, Toque and snowboarding gloves. I am sure it was a sight to see!