I'm thinking of using real water in my LEGO City and putting the Atlantis and deep sea exploration sets at the bottom of the tub of water, but I don't know what will happen to LEGO when put in water. And maybe having goldfish at the bottom
The Atlantis Submarine Voyage ride at LEGOLAND Windsor has almost 100 models (albeit glued together) submerged in a 1,000,000 litre tank with 'upto 50 species of rare sharks, rays and tropical fish'. If LEGO bricks can withstand those conditions, I think you'll be fine.
Most of these models are quite large (and heavy), so you may need to weigh down or attach some of your models to the bottom of the tub to stop them floating.
Unlike LEGOLAND, you won't have to worry about the upkeep of plants and fishes. I would suggest you replace the water every week or whenever you see it is getting too stagnant.
Apart from the traditional plastic LEGO brick, electronics and batteries should never be submerged in water. Other 'non-traditional' parts including Cloth, Foam, String, Paper and Rubber Bands may deteriorate more rapidly when submerged (depending on the water contents) and stickers will very likely lose their adhesive and peel off.