Sometimes it's salt, sometimes it's freshwater - honestly it has creatures from both and much like a lot of Night Vale, it doesn't really make sense. It's about a mile from the sandy beach to the cliff side, and twice as deep. There are benches and a few tables near the beach to have lunch at, and plenty of warnings not to litter. Regardless, your chances of seeing a half naked man (which half, and which man, really depends on the day) increases by 75% when at the lake. The landscape is always changing as both Poseidon and Hephaestus terraform with the help of Hecate. So if you've picked your favorite spot, chances are it won't be there next week.