We did a tri at the end of June in Denver 3 years ago (Tri the Creek), and the water was FREEZING. I remember they said the water temp would be upper 60's/low 70's- but they were wrong. There was no way it was that warm. Before the race start they said they'd had a lot of snow that spring and the water was cooler than normal. Uh- yeah.
It was a sprint, and although I got warmed up, it was COLD. I had a sleeveless wetsuit on, and I seriously don't think I would have done it if I didn't have one. I would wear one- or at least have one in case it's frigid.
I live in Oklahoma, so am not used to that cold, mountain-fed frigid water. I suppose if you're from that area, you might be more used to it if you swim in it regularly starting in June.