Commercial diving, versus navy diving...apples and oranges, as is said above, there's no guarantee that you'll get as far as being a diver in the forces (UK or US) and if you want to be a commercial diver, then waking up in a tent in Helmand would not be pleasant....
of of course most forces diving courses are intensive, in depth (no pun) and will not allow you to proceed if you are not good enough....but when you leave the forces and try to get a job, say, on a pipeline tie-in in quatar on surface gas, maybe, then there are 'children' who have been diving much less time than you that will get the job because they are experienced in tie-ing in flanges on deep surface gas jobs.
bottom line is, if you want to sign up and serve your country, go for it.
if you want to become a commercial diver, then go to a cheap school, and spend all your time learning about being a good commercial diver and keep at it.
(I always think this is kinda similar to someone who wants to drive trucks- do you go pass your driving exam, then your HGV, etc, or sign up to drive tanks for a few years first....)
oh, and yeah, if you want to be a diver after all this, then think long and hard...the downturn in oil prices is verging on catastrophic in the North Sea, and I can only imagine how it's going in the GoM, as I believe things were already in the ****ter for the last few years before this recent drop there. Boats are tied up, contracts cancelled or postponed indefinitely, and we are already seeing unscrupulous 'dog-****er' companies offering desperate divers work at over 60% less than the agreed rates for the North Sea. Now, the price has already levelled off, I believe, and we will see small increases, but companies want to 'streamline' so that profits go up, and when the price of a barrel increases, everybody's great again ('cept for the poor bastards that have been laid off). For most companies, it is not about loss as much as less profit. There's a big difference...