I went to a local water park last May when it was in the upper 60's and the place wasn't that crowded but walking around in a wet bathing suit was cold, so I only stayed for about 90 minutes, changed and went to the amusement ride side of the park. I couldn't imagine going to a water park in 55 degrees even though if I was a local, I might go, ride Summit Plummit a few times and leave. After all, how often does it not have any kind of line?
Personly I wouldn't mind swimming in that weather but I don't think I would going on the slides. I wind would make me too cold up on the top.
