The coldest I've done was 54F degrees. We suited up, me with a 6.5mm wetsuit, and then walked over 1/4 mile in early Summer. We did a pit cavewith light penatration. I was so hot from the hike I didn't care how cold the water was.
Also did a night dive at Lake Jocassee, SC in late October a few years back. Bottom at 110' was 56F...burrrr!