These are very large vessels with a lot of rail to defend and a 24hour watch to be maintained while transiting the Somali coastline. I was going to suggest posting a Marine Squad on each vessel. Pick the Marines up in say Kenya & fly them back to Bahrain for the next trip. Equip the squad with some M240's and a few LAW's rockets (to keep the RPG's at bay) and a sniper rifle.
As you say, gotta be cheaper than maintaining a picket screen the length of that coastline. And it would probably tie up only a battalion's-worth of Marines, plus some air & boat assets.
I'd add at least one, preferably two, M2 .50 BMG machine guns. They will keep the RPG shooters at bay even better than a LAW. The sniper rifle could of course do it as well, but with much more difficulty.
But none of those weapons, save the sniper rifle, is all that difficult to learn to operate with reasonable skill. Train and arm the crews. If the threat escalates, institute convoys, just like they have done farther north. A couple of frigates could provide security for dozens of ships. They just have to work together a bit. It's not like these guys have airplanes or U-boats.