Larry the heat exchanger HW heaters are pretty reliable. You already have the footprint and the plumbing. Tanks aren't that expensive. We are full time liveaboards and shower aboard anytime the crew don't care to walk up to the marina when we are in port..which is to say, frequently. Shore power heats fast enough to get everyone (2 adults, 2 kids) done in about 1 1/2 hrs from switch on to wiping down the head. Running the engine actual gets the water hotter and does well enough for the entire crew to shower at the end of the day. We use an Atlantic Marine 6 gal unit. Has a 5 year warranty and has worked fine. The boat originally discharged into the bilge with all the attendant problems mentioned. I tried unsuccesfully to use a Jabsco Par Max 3 Shower sump pump and after 3 replacements I went to a Whale Gulper model and problems ended. I use a Y valve that connects the shower sump to the "Drying bilge pump discharge line" works great. Might add that the crew is well trained in water conservation even though we have 150G. Also with a 4-107 we produce sufficient heat with the exchanger pretty quickly. Water temps are in the mid-upper 40's right now. Hope that helps.