I agree with Buenchico, eddie and others. Some careers require a degree in a specific subject ( eg a doctor needs to have done medicine or an actuary maths) but many employers of graduates just require someone of graduate calibre who has shown the ability to organise and apply themselves, to collect interpret information, solve complex problems, communicate persuasively or informatively, etc.
Also, graduates may change career several times- she may move into a field later where not only her skills but also her knowledge of marine biology or science in general will help her.
University gives students a chance to make their own way in life, organise themselves without teachers organising everything for them, find out more about themselves- their strengths, weaknesses, interests etc, make contacts for later life.
Of course there are some courses which on the face of it seem to be much less useful or demanding but I don't think marine biology will be one of them.