Technical trades, as you'd expect, require a relatively high overall recruiting test score, particularly in the maths and mechanical comprehension sections. There are minimum required scores, not maximums.
Knowing the exact score required is unlikely to benefit the individual unless you intend answering the first 66% correct then nipping outside for a tab and a tin of beer inbetwween each section whilst everyone else finishes off the section. Suffice to say, aim for 100%.
Submarine-wise, it's probably best to ask the ever-helpful submariners on the site as they will always give a helpful, cheery insight. Probably.
Generally speaking submariners get paid around £300 per month 'extra knowledge' or specialist Submarine Pay, once qualified, together with a £5000 (pre-tax) Golden Hello. You also get to save a fortune on suntan cream, and some (not me, of course) would say soap, too.
The things you have to consider about submarines is the disadvantages such as limited scope for overseas visits (although when they do occur, you are accommodated in a hotel when alongside)and the limited ability to communicate with home when deployed operationally.