I'm new to both RibNet and boating, but I do work in the electronics industry with interests in physics.
GPS devices all work from the same basic principle, though readings could vary - based on the way they attempt to integrate correlated satellite readings in software to a meaningful display (e.g. mean-average, median, peak-hold, etc). Only if the speed were sufficiently constant should all methods read the same, though I'd guess bouncing around on water could have significant impact in the result.
If it were me I would simply borrow a few devices (phones/software or other) and carry them on the boat, comparing readings at given speeds.
As for absolute speed, the relationship between power and speed isn't linear of-course, and there's power-to-weight to consider. Sounds like you're really motoring though, whatever the actual pace
All the best,