In a bit of a hurry just now so just a quick note; I made the transition.
I'm now 73 and went to sea aged 19 in 1962. I mention that to put any comments I make into perspective; things have changed.
Aged 15, Apprentice engineer - Aged 19, seagoing marine engineer for almost three years. Great for a young chap and, on banana boats, got to see places where the tourists didn't go.
By the age of 22, I had decided that, one day, I’d get married and was a bit concerned that a seagoing life was not an ideal occupation for a married chap.
I HAD thought, that, if I continued at sea, I'd go for liners/cruise ships for the more interesting social life (although, in those days, consorting with pax was frowned upon). We carried twelve pax and I recollect a young 3/O taking up with a comely wench. The poor guy was dead on his feet - remember, she could sleep whilst he was on watch
I'd thought that I'd work in a turbine generating station for which my experience at sea would have been suitable but, thanks to the Cold War and the failure of the two Ks to play nicely together, the RAF needed cannon fodder and I needed a career. (I occasionally raise a glass to Kennedy and Kruschev)
I had to get myself some O-Levels and worked in quality control at IBM until the exams came round and was extremely lucky to be selected for RAF training. Although I wasn't happy with the £5 per week I was paid as a cadet cf the £20 I earned as 4th engineer, it was a lot better than PTF.
My feeling? The sea was fun for a couple of years but I wouldn't have liked to make it a lifetime career. The other thing which I learned was that, at the time, an engineer could come ashore and easily find engineering employment; for the mates it was dead mens' shoes in pilotage, harbour master or instructing.
Nowadays? Either get into the mil, a major or an airline which looks likely to be taken over by one.
Re how to handle DIY training; the advice you get here from people in their twenties and thirties will be more up to date than anything I can say.
In the end it's your decision. My life has been a series of serendipitous accidents.
May yours be similar.
p.s. I think my previous as MN officer helped a little at RAF interview. I am sure that deck officer training would help even more at any interview.