So, I'm only playing devil's advocate here for no other reason than to explore the discussion from an academic standpoint, but ...
From a systemic (and perhaps diagnostic) position, one could feasibly also track heart rate while diving and how depth or various breathing gasses affect it. It could then be compared to hiking, running, or cycling, or being at rest. If one were to experience ... not problems, per se, but ... changes? while diving having one more metric could help pinpoint the source and how to handle it moving forward.
Of course, that would mean wearing the Mk2 against the skin i think very few folks here would be doing that ... which reinforces the argument against wanting a single device one doesn't have to remove for all activities. It still has to be taken off and affixed over exposure suits, so whether you're taking off the Mk2 and replacing once you're in your drysuit or taking off the Mk2 and replacing it with the Teric, not much difference there. (Maybe wear both. One inside the exposure suit. One outside. Heeheehee.)
I did address this point earlier. When diving, I think gas consumption and heart rate are going to directly correlate. And, gas consumption is the more important thing to pay attention to.
No, you can't compare gas consumption while diving to anything when running/riding/hiking. But, is that really relevant?