My GPS is 12 Channel WAAS, when it locks on in 3D GPS it's accurate to 3' so I can't vouch for other less advanced units..

One way GPS on a boat is a bit innacurate is that it gives you "land speed" and "water speed" can vary due to currents which are not factored in..

But we're talking maybe 3-5MPH Max currents vs pitot speedos which are usually 7 to 10MPH off and don't even register at 10MPH or less.