Hi Sam,
First off, I’d recommend a wide angle zoom lens like the 13×4.5 for shooting on a small boat. This will allow the close focusing and angle of view required in such a small place.
Now if you’re talking boat-to-boat or from boat to ocean scenics, I’d recommend a gimbal tripod. I used this once on a boat journey across the pacific with great success. I don’t know where you’d get it, but there are more than one marine specialty companies that could help. Basically the gimbal tripod has a heavy weight which dangles on a long rod in the space between the tripod legs (which are tied down or weighted to the deck) and the pendulum effect of the weight counters the rolling of the ocean and keeps the horizon level. Here’s a link with more details and illustrations:
https://www.luminous-landscape.com/reviews/accessories/wimberley.shtml
Obviously there are stabilizing devices too; perhaps even a steadicam, which operates on the gimbal principal. I’ve also used a Kenyon Stabilizer for hand held, but only with smaller cameras than what you specify. Here’s a link:
https://www.microsearch.com/rental/new_rental/pages/kenyon.htm
There are, as I say, specialists in this area, such as:
https://www.sfbayimages.com/marine_film_cordination_services.html
Who can be contacted for details on the latest technology for marine photography.
Bon voyage!
JS