What other A-level should I do to get on a marine biology course at university?

Look at the 'entry requirements' on relevant Uni course pages - they will state clearly the A level subjects that are essential or 'preferred'. If they only mention one or two subjects, the third subject can literally be anything - they just want that 3rd grade. So pick a subject you will enjoy studying and where you feel confident of a good grade - not just something you think 'looks good'.
Examples
Marine Biology (Hons) | BSc | University of Southampton - Biology and one other science
BSc (Hons) Marine Biology - University of Plymouth - Biology and one other science related subject
University of Glasgow - Undergraduate study - 2024 Degree programmes A‑Z - Marine & Freshwater Biology - Biology or Chemistry
Ocean Sciences BSc (Hons) - 2024/25 entry - Courses - University of Liverpool - two sciences

The one I'm most keen on is at Queen's University Belfast -
https://www.qub.ac.uk/courses/undergraduate/2024/marine-biology-bsc-c160/
It says "BBB including Biology and at least one from Chemistry (preferred), Geography, Mathematics or Physics".
I'm basically trying to figure out whether I should attempt another difficult subject out of these, or just prioritise getting BBB without making it such a challenge.

You only need Bio, and preferably also Chem - that's what its telling you.
It doesnt say 'must have 3 sciences' or '3 sciences preferred'.
Its worth emailing QUB and asking if you took another science instead of Chem, would this really disadvantage your application. And remember, you need to have 4 other UCAS choices as well.

I definitely want to do biology and geography. I know chemistry would be good, but I would hate doing it. I've wondered about maths or physics, although both are quite daunting. I'm interested in psychology and English literature a bit - I'm guessing they're too unrelated, and I'm less of a wordy person.

Hi vee.hmm,
It definitely depends on which university you're interested in doing marine biology at. For example, for the BSc marine biology course at Exeter University (Cornwall Campus), they only require you to do one science (link Here).
I am a recent graduate for marine biology (and really loved it!) and am currently doing a marine vertebrate masters still at Exeter and many of my friends did subjects like psychology, history, english lit, etc. In some cases, it actually helped them as it gives you that different perspective in some cases which others may not have e.g. in essay writing and things like that.
As long as you check entry requirements and the university allows it then there's no reason why you can't do a subject you're interested it! Hope this helps! - Alicia, University of Exeter Student Ambassador