I actually considered doing this myself, though ended up in the City instead. Not sure I'd bother with the degree unless you are being paid whilst you do it. If you started in the MN at 18, you'd be an OOW or the engineering equivalent at 21. You'd earn £25-30k for a couple of years then perhaps £35-40k, particularly on tankers. Tax-free if you're at sea for more than half the year I think, plus no rent so you could go for the airlines at 26 I reckon.
Tankers pay the most, pax vessels the least as far as I'm aware. It's not for everyone from what I've heard, though I sort of wish I'd done it that way myself. I'm either going to try and get a role similar to my own but as a contractor, or drive trains for a living, both pay well.