WARNING: I'm Brainstorming, and this is intended to bring discussion! Translation: Don't call me nasty names unless you have a better idea.
Businesses world wide have security of some level, and they pay for it. Some have "in house" security, and some contract for it. Our marine based military is the Coast Guard and the Navy. How about if the Coast Guard would have trained security squads that would be based on every U.S. Flagged navigating the open seas or any vessel that needs security (meaning in our waters).
The ship owners would pay for this service. However, the use of U.S. Military could get around countries that would try and deny arms in their ports. This is our military on U.S. Flagged ships providing professional security.
Someone like deepwater could give us some idea of how many it would take. But I would think that it shouldn't take more than six per vessel, and maybe as low as four.