The RN will give you up to 6 grand to spend in enhanced learning credits, you can use it in 3 x up to 2 grand chunks but each time you use it, if you don't spend the full 2k it's gone.
Eg. Course costs £350 or £2000, that batch of cash is gone, you are now left with 2 x £2000.
I have a friend who has done an OU degree and it pretty much took over his life and he found it extremely hard to study when deployed, I'm not sure he was prepared for quite how time consuming it was and ended up with a 2:1. I hope to start one next Sept.
Standard Learning credits are £175 per year, every year. Same goes for both credits - if you don't use all then it disappears and you can only pay for 80% of the course.
I did a Masters Degree a few years ago, whilst living in during the week. Need to consider the pressures of the day job at the time, so that hopefully your evenings are your own time-wise.
Also consider how assignments are to be submitted, as if relying on pusser's post whilst at sea you may dip out. Access to the internet is also obviously a consideration..