I guess I found a workaround to sims not swimming laps in pools.
FIrst I went back to the pool where my sim successfully swam laps the other day. It was actually the fountain at the Perfect Balance Spa that I turned into a pool. The measurements are only 7 by 3:

So I thought I would try a big pool. I went to Bathe De Rill in Windenburg. No laps. As described above, my sim attempted laps, but dropped the action immediately. No picture of that, we all know what that looks like.
So I thought I would try building a 7 by 3 pool at home as final proof that small pools work. But the same thing happened, my sim dropped the action! I remembered though that the pool at the Spa was built into the foundation, so I made a foundation and placed a pool on it. Like so:

That did not work either, my sim could step onto the foundation, but was waving as she could not get into the pool. THEN I remembered that the pool at the Spa also had a square of flooring next to the pool and I built this:

And finally my sim was able to swim laps again, without dropping the action! She kept swimming laps until she was exhausted again.
Surprisingly it did not work when the piece of flooring was at the long edge of the pool, only when it was on the short tile edge. It seems sims now need their pools to be just so.
I guess all of this will not be necessary if the issue is patched soon.