Could you post a screenshot of your daily heart rate graph from the concerned day?

That's not the daily heart rate, I really need to see the all-day HR graph for the same date as the the activity

Sorry, still not yet all what is needed. I especially need to see the Resting and High HR values that are under the HR graph, when in the portrait mode - they play the principal role in the Calories calculation 

... but anyway, even just by guessing the Resting HR from the all-day graph to be somewhere around 60 bpm, I think that you are right that the 80 Calories burned during 37 minutes seem to be pretty low.

Could you check on the second tab of the activity screen, where all the numeric values are, how many Resting and how many Active Calories were there?

Run

I normally burn calories so I'm surprised it's like this in the pool anyway, I'm putting the calories and values ​​of my running activity as an example.

Yes, you are right, the active Calories during swimming are rather low, seeing the time and heart rate data (including the Resting and Max HR). I recommend contacting the Support and sending them the data.

BTW, did not you use the "drill mode" for the swimming activity? I am not familiar with that mode, since my watch does not have it, but the article "How are Calories Burned Calculated For a Swim Activity on a Garmin Watch? | Garmin Customer Support" tells the following (although I think it only applies when you do not have any heart-rate data for the swim, which is not your case):

NOTE: Calories are not calculated on the watch while in drill mode. While in drill mode the watch is unable to determine the type of drill being done (kicking, sculling, one-arm swimming). If the watch is in drill mode the displayed number of calories burned may be lower than expected since it is unable to detect stroke type.