Okay.
I've only done a little bit of underwater stuff, using a pool. It can be done, but there are some things that might trip you up:
#1: Make sure that ALL of your cast are strong swimmers. You mentioned that the whole piece takes place in the pool. Do not even bother casting anyone who cannot swim, trust me. This seems to go without saying, but I was 1AC on a feature where our last day was all underwater work. The lead actress did not know how to swim, which made the day rather, interesting.
#2: Source your underwater camera housing immediately so you can test it. Make sure that you know how to properly seal it, and that it is the right size to accommodate whatever camera you are using. Bear in mind that when you place a camera inside a sealed environment you have effectively created a small convection oven. The camera generates heat (some more than others) which has no place to go inside the sealed housing. This means you need to know how long before an overheat (if you are running something that runs hot) so you can pull the camera from the housing before that happens. On the shoot mentioned above, our RED MX lasted about 20 minutes before risking a shutdown inside the housing. Might have been better if production had gotten a proper housing, but I digress.
#3: It would be interesting to test a high-iso, relatively low noise camera underwater using only the pool's underwater lights and ambient from above the water. There's a possibility it could work, but water eats stops of light, crystal clear swimming pool water probably not more than a stop or so? Not certain on that. I'm sure there are lights that can be taken underwater, but ones that can AND are designed for film making are out of your budget range. A little creativity with waterproof, high-power flashlights perhaps? Otherwise we're on to:
#4: Lighting the water from above is fairly easy, looks very natural, and just requires a LOT of attention paid to safety. The LAST thing you want is for a 500w light to get kicked into a pool that contains your cast, your underwater camera op, and your lifeguard.
You DO plan on having a lifeguard on set and a car on standby with directions to the nearest medical facility, yes? I wouldn't consider doing an underwater shoot without them. Safety First. Which brings me to:
Any lights you use need to be OFF LIMITS to anyone who is not your lighting crew. NO ONE walks around them to reduce the risk of them being knocked over into the pool. These lights will be on stands pretty high in the air to achieve a fairly classic underwater look (rays of light from above). This means sandbags, lots of them. All cables and stingers should be dressed back away from the water and preferably taped down even if you are able to keep people from walking around them. A good grip crew will also be able to rig safety lines so that if a light does topple, it doesn't hit the water.
Finally #5: If your story actually takes place in a pool, then you are golden. If you are using the pool to simulate some other underwater location (lake, river, Atlantis, whatever) then have big sheets of black pond liner plastic (Visquine in the states, not sure what it might be called in Oz). This can be used to create inky blackness around your set that implies a much larger body of water than a pool.
The mirror idea is interesting, but in the event you find you need lights, you can pretty much use any you can get your hands on, but you have to use them very carefully and safely.
