Shutter speed above everything else. The boat, you, the water and the subject may all be moving at once, so start around 1/1000 and go faster for birds, splashing wildlife, rough water or a long lens. Keep image stabilisation on, use continuous autofocus and burst, and let Auto ISO adapt as the Zodiac swings around.
Leave more room around your subject than you would on land. Horizons and framing are much harder to control from a moving platform, and a crop later is easier than a clipped wing.
Keep the lens hood on. It's doing real work against spray and knocks out there.
Two things that aren't about the camera: don't stand unless the driver says it's safe, and don't brace yourself or your gear in a way that blocks another passenger or the driver's view. Both come up more than they should.
For landscapes and icebergs from a moving platform, something between 1/500 and 1/1000 is safer than the slow speeds you'd use on land.