Endomondo tells me 740 kcals for my 4k swim today. I think it's wildly inaccurate though because it bases it on time rather than effort, so I could have done a lazy 3k in the same time and it'd tell me the same calories. Are there any green light HRM watches that work underwater yet?
I think the garmin is pretty similar - it may take some account of speed but it clearly can't tell how efficient your stroke is.
I've not seen any underwater HRM watches but haven't really been looking.


), I'm often struggling for breath after 4-6 lengths. If I can force myself slower it's a lot better though. I think I'd struggle breathing every 3 stroke rather than 2. So I'll probably just try doing a few lengths each side and swapping. Last time I tried breathing on the other side I got very confused and ended up drinking water 
Actually many pools ban this stroke from public sessions for that reason, but it might be allowed in lane swimming.