I actually stayed at the Hilton Frankfurt and swam in the pool there--several times.

The pool was actually quite nice, though it did get quite busy. I might have been able to start with my own lane, but I think every time someone else came and I had to eventually share. Thankfully it wasn't that busy that we had to circle swim (would have been quite difficult, since my German is a bit rusty, and with people of different speeds and such).

The unusual thing I found in the locker room was that the attendant wasn't necessarily male. Actually I think they were all female. No one seemed to raise an eyebrow while changing.