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Measuring the distance of an open water swim can get tricky. I personally use a Garmin 310xt for my triathlon training and open water swimming because the watch is waterproof. But I have found that the best way to measure my swim distances at the lake is to tie my Garmin on a string to the zipper on my wetsuit and place the watch under a swim cap. This seems to provide the most accurate swim measurements for me.
I've often referred this this blog and you might find it helpful, as well.