For years, I kept a 250 gallon saltwater aquarium. I found it quite relaxing to watch the fish do their thing. Unfortunately, when we sold the house, the aquarium went with it.
Since then, I have toyed with the idea of another aquarium, but this one would be quite large: 6 to 8 feet tall (that extra height requires substantially thicker and more expensive Plexiglas) and about 12 feet wide (probably with some kind of column in the middle, or the required thickness becomes unmanageable. I figure it would be 4-6 feet in the other direction (length?), and would be large enough that I could hop in to clean it. It would be saltwater, but maybe a "coldwater" tank, not tropical, so I could stock it with stuff I capture.
The problem with aquariums is that it really limits my ability to be away from home for more than a few days. (I know there are services, but ...). So, that forces a choice between the boat and an aquarium. For now, I am sticking with the boat. If only I could figure a way to put an aquarium on the boat.