It would be nice to see more tank decorations (not sure if that's in your current plans with this being an Interior Design based modpack). If it is though, a submarine (yellow or otherwise), perhaps a classic message in a bottle?

Maybe a a ships wheel would be a nice wall decoration, perhaps a water barometer? Posters seems to have a lot of scope/usefulness (tide times, species info, plastic in ocean, great barrier reef, bermuda triangle warning). A classic ship door with the locking mechanism or one with a portcullis might be interesting?

If you ever get into making tanks and other bits and bobs, would be cool to see a tank where customers can purchase food to feed the fishes in that particular tank, Maybe similar to the existing sales tank but with the additional requirement of a vending machine for the food which is where the sale is. Or perhaps I should be commenting this to whoever made the sales tank, no idea who that is only got the game recently :)