Final answer:
The main cause of fatalities in small vessel incidents is capsizing and swamping, due to overloading or bad weather.
Step-by-step explanation:
The major cause of fatalities involving small vessels is b) Capsizing and swamping. This usually happens when the boat becomes unbalanced due to overloading, bad weather, or improper handling. When a vessel capsizes, it turns over in the water, making it unstable and potentially sinking. Swamping occurs when the vessel fills with water, often due to heavy seas or inadequate bilge pumping. Both situations can lead to fatalities, particularly if those on board are not wearing life jackets or cannot reach safety in time.