What are 'Defensas' (Fenders) used for on a recreational boat, and where should they be deployed?

1. They are underwater fins used to stabilize the vessel when underway at high speed
2. They are pneumatic or foam cushions hung along the topsides to prevent impact damage against docks and other vessels
3. They are navigation beacons mounted on the masthead to warn of approaching storms
4. They are metallic shields attached to the bow to cut through dense seaweed