There was very light wind and slight waves the day of the sea trial. There was nothing being towed. We did run the test in both directions, and there was no current being in a relatively small lake.
The instruments and GPS both displayed the same speed, so I do believe them to be properly calabrated.
Thank you all again for your inputs...I am making a list of every possible solution anyone posts to go over with the surveyor, and will post here next week with what we find.
Please keep the ideas coming.