Apnea Hunt: no heart rate data from chest strap

I have done some freediving in the Apnea mode with the swim chest strap on and from what I remember the chest strap won't give you real time HR data DURING the actual diving which the wrist HR will. While saving the dive activity when we were getting out of the water we had to click on something like "download HR data" and the chest strap would synchronize all of the HR data it recorded during the activity to the watch.

Thanks dude, Im going for a spearo today so I will see if I can find that option when I get out. Fingers crossed and thanks again for taking the time to help

Sadly this hasnt worked, it gives me no option to download the HR data

This document could help: 

Transferring Stored Heart Rate From an HRM-Swim or HRM-Tri to a Compatible Device at the End of an Activity

Also read the associated document for some more details. The basic assumption is that you have paired the HRM with MK1 already before the dive.

You should also know the following:

"The Descent watches can pair with Garmin heart rate straps for swimming, such as the HRM-Swim and HRM-Tri. While Descent watches are designed for diving, the heart rate straps are rated at 5 ATMs and its use is not recommended for diving." [source]

At the end of the dive you should click "save" then it gives you like a summary screen. Click on the activity/dive summary (instead of clicking "done") and there should be a "Download Heart Rate" option.

If that doesn't work and you have already saved the activity you can go to "History" on your watch. Then go to "Activities" then click on the activity you want the HR data for. Scroll down and there should be a "Download Heart Rate" option. I think you have to be wearing the HR strap to keep it turned on during the transfer.