All these courses teach similar skills - zoology and marine biology cover similar topics but use different examples from the terrestrial and marine environment respectively. There are transferable skills that can be applied to either.
Wildlife and conservation biology would probably give you a more specific skill set related to conservation work.
My advice would be to look closer into the module content of these courses, see what topics are covered and whether they fit into the plan you have for yourself.
Email the school contacts with specific questions (they are busy and probably won’t reply to general questions like “what should I do with my life arrrgghh”.
Many ‘marine biologists’ come from differing background, it all really depends on what career you are aiming for in the field. Figure out what interests you, and try and match the modules that fit.