#2: The Belt Broke
You probably won't hear the sound of a drive belt breaking over the general engine noise, but you will know something's wrong when your overheat warning light comes on, or your voltage meter shows that the alternator isn't charging. Having a broken belt is a scenario unique to inboards and I/Os, and one that can shut you down in hurry. Without a belt intact, you'll have no alternator or water pump.
Solution: There's a lot of info out there on jury-rigging a temporary belt by using fishing line or pantyhose or some such. This might work, but wouldn't it be easier to just carry a spare, along with the wrenches needed to change it?
Prevention: Inspect, tighten and dress the belt. You also might want to check the condition of the pulleys' contact surfaces. Sometimes, corrosion can cause rough spots on the pulleys that will eat a brand-new belt in short order.
Carry Onboard: Marine tool kit, which includes everything needed for this and other basic repairs.