Training in Maritime Security: Thinking about problems

Someone is working on issues of terrorism prevention and the small boat threat, as set out here, which I assume is from the Homeland Security National Small Vessel Security Summit". One scenario:

An Al Qaeda-supported group targets the cruise ship terminal in the Port of Miami. Two dirty bombs (chemical or radioactive, not nuclear) are assembled in Colombia and shipped in a container to the Bahamas.

The terrorists acquire (unclear if bought or stolen) two 45-foot yachts in Miami and dock them at Bayside Marina, where they plan to detonate one as a diversion before attacking the cruise terminal. Their surveillance involves taking photographs while posing as taxi drivers around the port, driving across nearby bridges, and hanging out in a park across the water.

The yachts sail from Miami to the Bahamas, where the bombs are trucked from the container port to a marina and loaded. The bombs sit on the back decks, covered in tarps. The vessels approach Miami from the south. The first yacht docks at Bayside and detonates. When marine patrol boats at the cruise terminal move to respond, the second vessel detonates alongside several cruise ships at the terminal.

How would you prevent it?