Im probably missing some vital things but this is what I have together at the moment.
I'm forever adding things as when I do any work on the boat and can't find a suitable tool on board to do it and have to get one out of the shed I put one into the boat (within reason of course - fitted a new hatch this weekend and won't be putting a drill and jigsaw on the boat
Open / ring spanners
Various screw drivers
Long nose pliers
Spark plug socket
Selection of cable ties
Loads of gaffer tape !!!
Pull cord for engine
Spare set of plugs
Spare prop and fittings ( would hate to try and change at sea! )
Plus other odds and ends - wire, cable crimps etc