It is a Garmin Vivoactive 5. You are able to monitor your heart rate real time. Whenever I dive, I set mine to an Open Water mode and it tracks distance swam and the heart rate. You can sync it to your phone as well. When I go back and look I can see an elevated heart rate that corresponds to me gearing up but as soon as I dive it levels back out. Like I had mentioned before, I mainly bought it to measure pool lap data but I fell in love with it and wear it all the time. I don't monitor mine real time, it's actually underneath my wetsuit but I do look at it afterwards.thanks Tigerpaw, what Garmin do you have please? I think some of them just give you a reading AFTER you surface, ie retrospectively, and I am after real time monitor..
Also, I know that Garmin also makes dive computers but I am unsure if they have the heart rate function you are looking for. Hope this helps out.