Bose 901...really

Yikes. A highly reverberant room can be very tiresome and fatiguing. I can't even read comfortably while teh kids are playing in most pool environments ( the noise is just too much ) The problem with traditional sound absorption (lots of wooden surfaces) is that the moist environment quickly causes rot....maintenance issues will be high - it is already a good idea to disconnect this building from the main house.

California redwood works better than most woods including cedar but I am not sure you can buy this anymore (maybe second hand from a building being torn down?) Of course you can treat cheaper wood extensively but who wants to breathe all that stuff in!

The most pleasant sounding pool environments I have heard are with a tarpaulin tent like roof...this allows the reverberant field to escape out the roof rather than being reflected back in.

you have good speakers in place why not use Dynaudio or nht instead for a better match?

Shadorne...Based on similar pools I have visited, humidity will not be a problem. This is a small exercise pool, 8X20. The building is well ventilated, and has AC for dehumidification. And it is a pool house, not an audio room, so the sound will be whatever it will be.

The Bose idea occured to me precisely because the room will be so reverberant. These speakers are based on reflected sound, and might be just the ticket. Does Bose offer a home trial period?

I would find a good used pair of the Bose 901 on Ebay or here on Audiogon and give them a try. Why not? If they don't sound right in your pool room, you can always re-sell for minimal loss. Then again, they might be just the ticket for a fun time in the pool with good sounds.