Perhaps the best items you can have onboard if your engine fails are your owner's manual and a cell phone. If you can't find the problem and how to fix it in the manual, a call to the tech-support line may help. Aside from that, the following items can get you out of a bind.
1. Screwdrivers: (Phillips and straight) These tools are essential to have, for obvious reasons.
2. Allen-wrench set: Keeping this onboard can make life a lot easier when it comes to removing certain types of hardware at sea.
3. Spark plugs: A stuttering engine may suffer from fouled plugs, so have some pre-gapped spares on hand.
4. Socket-wrench set: A must-have to turn frozen bolts.
5. Spare fuses: Spade-type fuses are critical on many modern outboards, so carry a selection of the sizes and amperages you need.
6. Adjustable wrench, Vice-Grips and channel-lock pliers: All are useful for turning stubborn nuts and screws.
7. WD-40: If parts are seized and all else fails, spray some WD on it.
8. Duct tape: It repairs cracked fuel lines temporarily, but you'll find many more uses for it.
9. Flywheel cord (where applicable): Smaller engines can still be started with a hand-pulled cord if the starter unit fails or the battery dies. Who would have thought that a simple piece of rope could be so valuable?
10. Spare prop: A spun prop is never fun, so you'll be in good shape if you have an extra onboard. Just be sure to carry a tool to remove the prop nut, as well as a spare cotter pin in case you lose the original.