I've just recently left the Merchant Navy after too many years, culminating in Captain with Stena Line on the infamous HSS.
I now fly for a well known big (regional) airline and after a year on turboprops I'm converting onto the 146.
My advise is to do the MN thing if you can't get airline sponsorship, but as the others have said , use the money and time to study for the ppl etc. You can't beat the experience of travelling the world on a ship and you will find that the academic side of things will be almost repeated in the ATPL's. The downside of the life at sea is that it takes a lot longer to get onto a big wage, and a Captain of a ship is on a lot less than the Captain of a long haul jet.
All in all, I don't regret having been at sea and I'm enjoying my second career unlike some of my friends who have another 20 years crossing the Irish Sea....