When I started out I jumped in at the deepend. It was a baptism of ice not fire, as it was mid winter.
The best place to start is on this forum, try searching for the things you wish to know about, and then start new posts with your own questions.
Re. Fish, I am afraid that the amount of juice the heater would consume would become a durge on your life. You would have to keep your batteries topped up frequently and know that there was always enough power for you to leave the boat for a few days. WIth a fridge if you run out of battery juice then your beers get warm, with living fish well...
There are lots of compromises that have to be made when living on a boat, probably the first hurdle is electricty usage.
Tim