Pier Fun O' Won was basically a bunch of houses built on the water and it has a large swimming pool. The trick is, you use foundation starting in the deeper water toward the shallower water/land, the same as if you are building a port. You can have solid, brick, foundation, which is what I used for my UC pier, or now you can use the new pier foundations. In either case you can space your foundation placements with gaps of four squares which you then just cover with floor, or you can leave it solid like a basement. (Don't think you can open it like a basement though. Haven't tried it yet.I'm thinking it would flood with water.)
Once you have the "pier", which is your house foundation, you just build your house and pool. You can use pool ladder to go in and out of the pool BUT you CANNOT build any kind of stairs or ladders down into the ocean or river water. What you have to do to get stairs down to the water is use the terrain tool to raise a bump of land, ( I use the largest square), then build your stairs to the that. Do it in short increments so you have a tall "bump" with stairs, then another, shorter and so one till you are ALMOST at water level. The last "step" will just be the lowest bump of land which will be under water.
In my first version for my UC pier, I used glass windows in my pool and seascaped both the exterior of the pool in the ocean and the interior and used fog emitters for fish and bubbles. It was pretty cool looking. With the new stuff available you could make it really awesome looking. (Note to self: try this idea out.)
