(I've just tried posting my reply and it's not shown up. I apologise if this appears twice):
Download the BES's careers booklet from here:
http://www.britisheco...ntal%20management.pdf
You'll probably need to take a post-graduate course, so it will be important to choose a relevant course for your first degree. A general course in Environmental Science (followed by a postgraduate specialism in marine issues) might be a possibility, but a more direct route might be to study Marine Environmental Science at Portsmouth Uni. It's available as a 3 year or 4 year (extended) course. See here:
http://tinyurl.com/ycbvksd
and here:
http://tinyurl.com/y8mckto
Those links will probably suggest which A-levels you should take but I'll specifically mention that you might find Maths useful. (When I was studying for my maths degree I was constantly having to help biology students who found themselves floundering with the mathematical content of their courses). However you should only do A-level Maths if you really enjoy the subject and you're exceptionally good at it. I used to teach A-level Maths and I found that even those students who could have easily have achieved a GCSE grade A, without any revision whatsoever, at the end of year 10 (if they'd had been entered for the exam) suddenly got one hell of a shock when they encountered the content and (particularly) the pace of the A-level course.
Chris