It would seem to me that unless the hijackers are the suicidal types willing to scuttle the ship while they are still on board that the Navy SEALS should be able to hijack the ship BACK before it makes port. Maybe this is a setup job to showcase the talent of the Special Forces?
The Brit Hostages present a problem of course, soon as you bring in the SEALS for a boarding party, Hostages will be shot and pitched overboard. However, which is worth more, the Oil or a few Brit sailors?
If they do let the ship make a harbor that will fit the draft and it drops anchor, then there is about no way you could get the ship back without either paying a huge ransom or mounting a huge military operation, and chances are the Somalies would then scuttle the ship before you could get it out of the harbor.
There is a decent possibility this is a covert CIA operation designed to get the Saudis to pay us to be their Sea Lane Police Force. Pay off the Somalies to hijack the ship, which then convinces the Saudis they can't sell any oil overseas to anywhere without our protection.
In any event, I can't see Supertankers taking any route that passes within 500 miles of Somalia without having an armed escort at this point. Without the ability to safely export their Oil, the House of Saud is up a creek without a paddle, so they will have to either pay Protection money to the Somalies, or pay Protection money to the US Navy. So basically, we can charge the entire war in Iraq to the Saudis as a Gangland style Protection scheme. We no longer will have to pay taxes to fund the US Military, the Saudis will have to pay the bills