GPS speeds are a pretty simple distance/time calculation... and erroneous results are easily explained. Most units track position at fixed increments of time, default is usually 10 to 30 second intervals. When you export your track and look at the table of data, you can identify where the errors are introduced. Here is an example from a recent bicycle training ride... Notice that for one interval, the unit lost reception and placed my position with no movement and then calculated the speed as double when it found the position in the next interval. This often happens under bridges or very large trees. On a boat it can happen when your handheld is blocked by your lifejacket. You will notice that the odd max speeds are an even increment of your expected speed on a boat for the conditions... 2, 3 or 4 times expected.

391 9/21/2006 19:02 909 ft 299 ft 0:00:10 20 mph 111° true N40.12869 W83.15433
392 9/21/2006 19:03 902 ft 312 ft 0:00:10 21 mph 109° true N40.12839 W83.15333
393 9/21/2006 19:03 901 ft 309 ft 0:00:10 0 mph 109° true N40.12839 W83.15333
394 9/21/2006 19:03 898 ft 303 ft 0:00:10 42 mph 114° true N40.12781 W83.15124
395 9/21/2006 19:03 891 ft 288 ft 0:00:10 20 mph 119° true N40.12748 W83.15025