And that's very much the case with any swim device. Invariably it comes down to the swimmer's technique not conforming with the parameters the device expects for that particular stroke. Despite not being a particularly good swimmer best is around 1:40/100 on a good day, i rarely lost a length with the Garmin 910 and am seeing the same with the 920.

I suspect the same will apply to the V800.

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Trust me I’m a doctor!
Well, I have a PhD :-)