I love the sound of this but the only problem i can think of with a fish tank map, whether its completely water or if theres land, is just the fact that to swim down you have to hold down shift, or whatever key you use, and that really does get annoying holding down one button to explore
like i made a brick layout on the volcano map i downloaded, and i made an underwater aquarium in it but the only problem is you float to the top, and swimming is okay in blockland but you just don't seem to have enough maneuverability