If you have dual mode IS, definitely switch on the mode that disables
the horizontal movement correction.
When the weather is such that the boat is slamming, I have simply gone hand-held with full IS. Use a harness, keep the knees and elbows in 'flex' mode, and avoid the use of any viewfinder to the eye since the camera will simply bang on your face
Sometimes suspending the camera with surgical tubing from a convenient overhead tie-off works pretty well - or use a helmet-cam!