I have been diving water that has been everywhere from the mid 30's in march under the ice to 40 degrees out in the Grand Traverse Bay of Lake Michigan just last week... Actually I have dove 40 something degree water 2-3 times a day, everyday for the last week and a half... I just did a two hour rescue review course this morning in 47-50 degree water, then went and did a wreck dive in 44 degree water, then another dive in 41 degree water right after that... I always dive a 7mm... I don't believe in dry suits, I think they are sacreligous, actually I just don't want to spend the money for one, I like the way the cold feels, it slows my breathing way down and calms me.
Anyway, just thought I'd share... Cold, IN MY OPINION is anything under 75 degrees, unfortunately, I don't get to dive warm water, but someday I am going to move someplace with 80 degree water year round... Hellll Yeahhhh....