More details needed. What island and what kind of boat - sailboat, zodiac, catamaran, large tour boat? Zodiacs are tricky as you can easily get sprayed in transit and they can feel like you are in a rodeo if the captain opens it up to full speed. If you have a lot of folks around you, a large lens is going to be awkward to move around. I have done several our of the dry side of the island of Hawaii on different vessels and did not have great luck on close shots. If I went out again, my preference would be a sailboat or a larger tour boat that did not pack you in. The zodiac sounded fun, but the risk of getting sprayed in transit was a concern as there is very little storage except what fits in the little well beneath your tiney seat cushion/saddle.
Have fun,
--Ken