Logging beach exercise

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So I'm going to the beach on Sunday for father's day. I usually spend a few hours (2-4) in the water just treading water and splashing around. Nothing too vigorous, but enough that I know I burn -some- calories. I don't have a HRM, so how can I log my exercise? Maybe cut my minutes in half (120 minutes of treading water with moderate effort (at the beach after all) is like.. 1600 calories... that can't be right) or something?

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