Two very boring additions: eye protection and disposable gloves (nitrile); I know, I know, but if you were to get something in your eye when off shore then the nearest A&E is a long way away, and keeping hands clean is easiest with a pair of gloves or 10.
While it's easy to make complete lists, essential might also refer to ''if you could only have one tool, what would you bring?'', and if only 2 tools? if 3?...
If only one, for me the Leatherman, I always have it on the waist belt and can usually do a half of the small boaty jobs, before needing to go for other tools in the big boxes.
Two allowed, I'd add an adjustable spanner.
Third, bit holder with flat hex pozy etc bits
Four ..