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.
Basic LEGO elements won't get damaged by water, however depending on the condition of your water source, some residue may build up on the elements after prolonged immersion. So if you are thinking about keeping them immersed for a longer time, I would suggest using distilled water.
The LEGO bricks themselves will be fine, but if you're adding fish and turning this into an aquarium setting you may need to perform some periodic maintenance on them to keep them looking nice... but lots of people have used LEGOs for aquarium decor with great results.
Not much. LEGO specifically designs some models to float, using a solid hull piece. Unless you build a water-tight boat, the pieces will sink, but they'll be fine.
I still have my cargo ship LEGO from 25 - 30 years ago. It was designed to be immersed in water. My daughter now uses it daily as a bath toy.
As long as you don't want an air bubble, it should be fine. Normal bricks do not create an air tight seal without glue.
Nothing will happen except that some pieces might float. By the way lego is waterproof, so it's okay to keep it in water.
Personally, I have dunked all the LEGO models I created (Ships and planes) into water to see if they float properly, and besides that water gets into every hole, there is not problem with LEGO bricks and water.