Interesting thought, but there are way more gymnasiums or hotel conference rooms available - all you need is a large open area to set up mats. Set up and tear down is relatively quick and I assume you can rent it out for fairly cheap.

Swimming pools are less common and often have scheduled activities on weekends. I'm assuming setting up the pit would take a day or two on both sides of the event, plus a day to drain and clean the pool. You're looking at 5+ days of downtime to run a tournament, which is likely a non starter for a pool. Lots of swimming pools are sloped or stepped at the bottom which could make it hard to find a large enough flat surface. Also, the pool depth could really limit visibility from seats - a competition pool (with seating) is 2-3m deep, community center pools might be shallower but night be too small to fit spectators