July 2017
I had the same issue, my sim was just treading water and couldn't get him to climb out or use the pool ladder. I tried resetting him with the "testingcheats true" code but that didn't work. I went into build mode and moved the pool, moved him to a different location, and then moved the pool back. I also had an issue with placing tile around the pool.. a row of tile would disappear when placing 2x6 tile.
September 2017 - last edited September 2017
Same here. As of recent, on some community lots (but not all of them), my Sims cannot get out of the pool. The pools seem to work fine on lots designated as public pools, but other than those, they always seem to get stuck. Resetting them doesn't work, and I have to zoom out and travel in order to free my poor Sims. This was never a problem in my game before, which makes me suspect that maybe a more recent update could be responsible.
September 2017
This bug is some months old and can happen randomly in every pool. On my computer the Sim will finally leave the pool if I wait 2 or 3 real life minutes.
April 2018
This bug has been fixed in my game for a longer time now.