Just got done looking at charts of that region. A vessel transiting through there will be in danger for two to three days. Longer if they're slow. Paying for an armed force, at union scale, for the duration of a whole voyage isn't cost effective, because you would be paying a team of about 12 to 15 people 24/7 for weeks! Plus food, laundry, benefits etc. Quarters for the team would be a problem since most commercial ships have limited crew quarters. Also many countries don't allow armed merchant ships in their waters or ports. International/Maritime treaties would have to be changed. I don't have a traffic count through there but it has to be one of the most heavily traveled regions on Earth. That would make putting US military teams onboard very expensive. Best way to do this is Private Enterprise, contracting with shipping companies for groups to board in the Red Sea and off load off shore near Kenya. You would have to rotate teams on a regular duty routine, say two round trips, and two weeks off.