I question the accuracy at those very low speeds.
This is an excellent observation.
Last night I did a little research on GPS accuracy.
I found most manufacturers claim a +/-0.1 MPH accuracy, but when tested they in fact show a more typical +/-0.5 mph accuracy.
Now, I still feel what the person asking the question is looking for is a unit that actually displays a reading below 1.0 MPH.
AND, I'm going to read between the lines here and say this person is really looking for repeatability in his display of a certain trolling speed.
I don't care if I'm doing 1.9 mph (or t#$ MPH).
If I'm catching the target fish at 1.9 mph, I want to keep trolling at 1.9.
I own a few GPS units and have only had a chance to test one of them to see if I can get a reading below 1.0 MPH
I do know that my Lowrance HD5 Gen2 will go below 1.0 because I have used when drift fishing.
Sooooo.............
Not sure if you wish to spend that much cash just to measure speed.