Saltdean Lido has been restored by volunteers who formed a community interest group to save it from developers. It’s incredibly expensive to run a heated outdoor pool, and it’s been incredibly difficult to restore the lido building as it’s listed and was in very poor shape when they took it on.
The dog weekends are hugely popular and make money to help subsidise the rest of the season. As PPs have said, they run them at the end of the season when the pool would otherwise be closed, and the pool is cleaned before reopening to humans.