Subbing to follow along. I've spent the last 18 years at sea on and off again, my wife thinks I'm nuts that I want to buy a boat when I retire in a few years.
I don't know what level of continuous water change you are after, but for energy conservation, you might consider a large influent holding tank which is well insulated and heated and slowing the exchange rate for the DT. This would allow less requirement for the heat exchange system to run continually. Additionally, don't let the warm cooling water from the diesels go to waste when running either. Add a heat exchanger of some sort and let that warm water take some of the energy demand off the in-tank heating.
Water parameter stability is a big benefit in a reef system, so the changes in salinity, nutrients, etc may play against you. I like the idea of using NSW, but if the parameters of your NSW are constantly changing, continual water exchange may pose less than ideal.
A low power heat pump may be an option for heating as well, the efficiency would probably beat that of electric heating elements if there is a low delta between local water temp and tank system temp.
Also, you have a lot of inertia and sail area, perhaps a hydro turbine generator to supplement the system while in motion.
For the heel, before reading much of the thread, my thought was to have the vertical height of the tank at least 200% of the visible viewing height. This would allow for at least 45 degree heel with some margin for error.
For the fish, I wouldn't imagine that any motion of the ship, whether heeled over or being pitched by sea state would have much effect on them. They get pushed all over the place in tidal areas by the swell.
From a submarine background, my perspective is on minimizing the electrical load requirements to maximize battery life. Then again, I suppose a pressurized water reactor plant is beyond the scope of your build.
At any rate, definitely a cool idea full of logistical and engineering challenges. I had the same thought on a much smaller scale in a Sabre of some sort, but I think the notion of maintaining both a reef and a boat would result in a new found single (i.e. divorced) life for me.
