I'm a fan of distance swims, we're going to a 24-hr swimathon next month, last year OH swam 25km and this year I'm going to see just how far I can go myself. Having learnt from last time I won't go until late evening, there are too many swimmers before then but from about 11 pm until around 6 am you can swim without hindrance. After that the early birds start arriving back in the pool. I can't deal with 20 swimmers per lane, mostly doing breaststroke.