Yep; geometric distortion. Thanks for taking a variable out of the equation.
Hard to find a lens that has zero geometric distortion. This is indeed more than others, and less than some. Generally speaking, the wider the angle, the more the distortion. To correct this kind of distortion, manufacturers add lens elements to the lens, increasing weight, complexity, and price at the same time. I'd have no reason to expect a higher quality out of such an inexpensive camera. After all, it is in essence a webcam mounted in a fancy gimbal. I don't expect the same quality pictures from it that I get from my DSLR with a $10,000 lens, or even a $600 lens for that matter.
What is the format for final use going to be? Personally, I shoot in 4K with the CGO3+, but only because this affords me to use a DVE 2:1 zoom without losing resolution, as I have no reason to ever use 4K as a final format. 1080p is it for me. In doing this, it would further eliminate any geometric distortion as the field-of-view of the shot is decreased.
I have yet to shoot near big water, so unfortunately I do not have a picture to be able to compare geometric distortion on my CGO3+
Best of luck in getting to the bottom of this!