I have also never seen it at 70 degrees. Water temps. are around 75degrees with very little variation. What makes it feel different (perhaps cooler) than in open water is that you have a physical ceiling above you, so no sunlight warming you up along the way. Also the dives are performed in a slow, very controlled manner, so you're not moving as much. As far as changing tanks in the water, i think that's a crappy practice that should be eliminated. People should have enought time to do a surface interval, go to the bathroom, drink some water, etc.... But that's besides the point.