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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.