Bruce makes a good point about staying low. The reason that works is your backgrounds and out of focus areas have more separation and are less likely to pick up alternate subjects.
Whether that works depends on your ship. My ship was a bit taller above the water and we spent 6 consecutive nights on board. I found little consistency as to the location of birds relative to the ship - while some were viewed behind the ship, it was pure coincidence and just as many were viewed from each of the other sides. I ended up spending most of my time higher at the front and sides so I had a wider field of view.
Pelagic birding and bird photography trips are a lot of fun but very challenging. One technique you might use is to program a button to switch to DX. That gives you more ability to see the subject when it is distant and small in the EVF. More importantly - it's a quick adjustment you can make after finding the bird in the full frame viewfinder.
It's a lot better to have a couple of lens options than to have the wrong lens. It only took me a few hours on deck to decide my longest lens and a tripod were the wrong tools. I was struggling too much to find a bird in the viewfinder. I was very glad to have something shorter and wider - to allow me to photograph something rather than missing the only bird I saw in an hour long period.
We saw lots of birds near land - leaving the harbor and returning to the harbor. But those birds were relatively ordinary. The exceptional birds were sea birds that did not approach land.
One more idea if you are serious about birding. I used a Nikon GP-1 to capture GPS coordinates in every image. We had the ship's electronic log which recorded the GPS coordinates throughout our trip and could be used to match the time of the image to the location, but having the embedded coordinates in every file was a lot simpler. Of course, we were outside cell phone range so that tool was of no use.