Hello Nicholas and Ulrik....
I hope to be down on the boat this weekend...maybe I can take some pictures and measurments of the area under the cockpit sole. I have a Yanmar 2gm in the Dreamer...all the water pump and such are accessed through the front cover under the steps. The shaft seal and tranny are all inside the engine bay and accessed through the starboard locker removable panel ...after the rear engine cover is removed. The area I'm considering would be aft of that, where my muffler resides. I don't think it would complicate any service work as there really isn't anything there...just empty space. I'm only thinking about 3 to 5 gallons, so it wouldn't be a very large device.
I really don't want to add much weight to the rear of the boat either. I definitely didn't want to add any weight to the starboard side as the boat already list that way as it is, and if I'm out...I need all the space for anchors and gear.
The rear of the engine cover has some access holes that I could run the hot water lines to the tank through and being located there they would not have to be very long and not lose much heat before reaching the tank.
So far it's just Ideas...maybe this weekend I can actually start looking for a correctly sized ice chest and tank to make it work.