No, it’s not safe. Diving has its own exercise demands, and doubling up on exercise is setting you up for bad things.
The bubble/bends concern aside, what defines exercise might be dependent upon an individuals general fitness. For a person who is tri-fit, recreational SCUBA diving would be an activity but it would hardly be exercise for them. Unless you are in panic mode your heart rate is not going to be in the target zone. It does burn calories though as the body has to keep the core temperature up in the water. Walking to the mailbox might be exercise for some but I doubt a triathlete could get much benefit. I guess if the diver were to jump in and start swimming hard for 20 minutes, hard enough to get to the target BPM and sustain, well, that would count, but who does that? And in a quarry? N