When using Watch7, why does "rowing distance" frequently record only about one-half of my actual distance?

I just bought a Samsung Watch7 and have used it on the water to track/record my distance rowed. Multiple times now, the distance recorded after I finish and save the workout is approx 1/2 of the actual distance. The GPS track/map of the workout is accurate and complete, but the distance does not agree with that track, is off by a factor of 2. (NOT a factor of 1.6! I'm not confusing metric/Imperial units.)