Belmont Shores, Long Beach There is a group that swims at 2nd street in Long Bch/Belmont Shores on the weekends Los Alamitos Bay (Long Beach). It's not a lake, but it is a protected open water area (sort of a lagoon or marina). The area is lined with buoys for swimmers, rarely has a current and best of all, no ocean waves and no ocean wildlife. It is the ultimate open water swimming experience that most simulates a lake.
Lake Mission Viejo
Castaic Lake (From Memorial Day to Labor Day)
Thanks to the LATC members who replied to me and assisted in the preparation of this info