Another reason the GPS based versions are kind of tricky is if you're dealing with any kind of current. I have lived on the mobile Alabama basin and now on lake Houston. Both of which have current, not necessarily on the main bodies of water but on the little rivers and such that feed these bodies of water. For example the San jacinto river feeds lake Houston and that's where all the wskeboarders go because its 100 yards wide so its protected on both sides and the water is usually buttery smooth. Now let's say you are pulling upstream and the water has a 2-3 MPH current under your feet. You set your GPS unit for 22 MPH and the relative velocity under your feet is actually 25 MPH headed upstream and 19 MPH downstream. A "paddle wheel" unit doesn't care about current. Just my personal experience and ultimately my personal $0.02.