How is the marine perimeter of a coastal airport secured?
Often time there will be a fence installed as is the case at Maho Beach where people line up at the fence to get up and close to takeoff. The fence appears to have what looks like barbed wire on top. 

If there is not enough clearance markers in the water may denote an area for boats to stay out of. Such is the case in St Barts. Aside from creating something a prop can snag these provide little security. 

Assuming its a controlled airfield there is a good chance the controller in the tower can physically see the coast (with binoculars). If you floated up in any boat of size there is a good change you would simply be seen. 

If you are talking about the departure end of the runway, it can also be difficult to just walk up to the back of a spooled up jet.