hmm, i'm not alone with this problem. . . . . .
Is it really a manufaturing-problem?
If it is so, I hope Sealife is solving the problem (with a new lens) and recall the bad ones.
I suspect that lens is made to work with more than one model of camera. On some it works perfectly as you would expect, and on others it will have vignetting unless you zoom in a notch or two. Very common with wide angle lenses.
It's not a problem with the lens or with the camera, just a minor issue with the way the two work together in this specific pairing. So I don't expect Sealife will change anything unless they decide to make a new lens just specifically for that model camera.