Google Maps

A friend of mine and I go ice skating on many frozen surfaces.  A year or so ago, I went skating on a slough with another couple.  They had a GPS and recorded 10.1 miles from the start to the turn around point.  My friend said he used Google Maps to measure the distance and he said it was 7 miles.  I asked him if that included the many curves of the slough.  He said yes.  That is a very big difference.  Is Google Maps accurate for measuring a meandering body of water on a map?  

Thanks
GB