Spare prop/props. Nuts and spacers for the prop. As noted wrench (also torque wrench with appropiate sized 1/2" drive socket. Spark plug wrench--and in some boats a set of plugs. Spare water pump impeller.
JB weld, various sealants and quick set epoyx. Duct and almaganating tapes, vinyl tape. A huge variety of fuses --all three sizes of spade type and an assortment of tubular. (not only for my boat, but there is always someone else looking for a fuse). Nuts, screws, bolts, SS threaded rod, in 3/16, 1/4 and 1/2". Head rebuild kit. In some boats we have either a water pump rebuild or a second water pump. Extra lengths of hose, and wire. (various sizes). Hose clamps, including those for the outboard fuel lines--as well as a spare primer bulb--also extra anti siphon valve. Full compliment of electrical connectors and good crimper. Adhesive shrink tubing.
The usual tools, including a strap wrench, pipe wrench, hammer, hatchet, Battery or 110 V operated Dremel tool--with various bits, including drill and cutting bits. Some extra pieces of fiberglass and epoxy for hull repairs. Plastic sheeting.
Good idea on the alternator belt. I'll have to look into that.