Man it's a tough one, but I think it's possible to accomplish a good result.
1. For the acoustic part, I would definitely ask from the production to cover a big part of the ceiling with hanging blankets. For the close ups I would use a second layer of blankets, above the speaking actors and a third one on the floor, and a 4th one blocking the sound coming from the walls or windows, so you can diminish the reverb in the wanted area.
2. Now for the drain issue, I would ask from the production, for the close up parts, to lower the level of the water enough so I doesn't come out, provided that the pool behind the speaking actors is out of focus, and you cannot tell that the water level has changed.
3. For the safety part of the wirelles system I would use the Aquapac 158 or 554 depending on which system you'll use.
4. For lav mics someone suggested to me the VT500 water, which can be sumerged temporarily under water...don't know exactly for how long...
5. As for the boom mic, I would try out a cardioid mic, or perhaps with the treatment you could use a shotgun.
6. For the crowd walla I would plan that part previous the shooting with the director. According with the shots and how the story evolves I would record specific sounds, like for example, just people talking normaly. Then more excited and then cheering. You could record a couple of people whisteling and cheering for their team or swimmer individually, and then of course the applauding part using the same variations as the vocal ones. The sound designer will appreciate that.
It surely sounds interesting and a challenge one. I would like to see how it goes. Good luck.