Long enough title for a post, right? lol Anyways, Graham has explained the technical issues that occurred when they tried to have swimming in ponds/lakes on lots.

The bottoms of lakes on lots have very uneven surfaces that make Sim routing very difficult as we’re constantly measuring the depth to see what the Sim should be doing. If you sent a Sim to swim in a lake on a lot, you’d see weird issues where they’d appear to be swimming in weird paths to avoid areas but you wouldn’t be able to tell why, or they’d start wading/walking in the middle of a lake. The other issue is that the shoreline for lakes on lots is a lot less predictable than ones built on the world lot, making it a lot more difficult and animation intensive to get a Sim into the lake in the first place. It’s a problem that we examined a number of potential solutions for, but there wasn’t a good solution that would account for the majority of scenarios to make Sims swimming in lakes on lots look decent.