I've no idea how accurate a watch really is for calories. It's having to make some assumptions, but it may show how your efforts vary. I may try my watch in the pool to see what it shows.
I'm trying to do some swimming over the summer in our little pool as a general exercise. It can work the whole body. It's also nice to cool off a little when it's getting warmer here. I can swim when it's too hot to run.
i think that all of the fitness heart rate monitors are going to be making some guesses. The other day I was just walking somewhere and my watch told me my MAX heart rate was over 150, which simply wasn't possible because I don't remember breaking out into a full sprint during that casual walk to my buddy's place.
I get some iffy readings at times. The GPS on my Garmin is pretty good though. I used to get glitches with an old app I used on my phone (Endomondo) and it would show me doing crazy speeds.
those crazy glitch speeds have completely screwed my overall time records on strava. they are too numerous to delete manually without spending all day on them. I also have gotten some pretty bogus ideas about progress because the GPS would jump me across town for a microsecond as well. Annoying but I guess that is what we get for a hundred dollars.
I've hardly had those issues with my Garmin and Strava. It was much worse with the Endomondo app and the GPS on my phone.