Never on a boat, but then i've never had a boat prone to snaking (havent tried new set-up yet... hope it's not cursed now)
These devices, however, do work... they transformed a large box trailer (pic attached) that i built. At first you thought you were going to die at about 38mph... like both lanes of the road dragging the Range Rover all over the place. With TWO stabilisers fitted i can get upto nearly 80mph without problem.
That's a big, heavy (3ton) trailer though - would not have thought a boat should need it unless it's a REALLY big one (Charles, Brian etc.) and you can't tow anything that big in the UK anyway.