60' is a boat, not a ship!
There are many more movements in different directions.
As many stops of IS as you can, preferably dual, lens and body.
Safety lanyard (can be home made from 6mm "paracord") not attached to the strap fittings, attached to your belt
Sturdy ziplock bags that will cover the camera, with a hole cut in the front that fits tightly over the lens. (you can get special bags with hard plastic to go over the lens, but I doubt the optical qualities) Although the camera may be weather protected, salt seems to get everywhere.
Proper deck shoes
Insurance
Don't hesitate to use high ISOs, noise doesn't seem so visible in bad weather at sea, and some of the nicest shots I have seen have been taken in bad weather
Chamois leather and microfibre cloths in sealed bags in your pocket.
Rain suit, wrap around glasses, and a secure hat that stops the water going down your neck.
I'm assuming it won't be sunny, holiday maker river cruise weather. If it is, then you will have nothing except photos to think about.