Science majors are hard, and if you are interested in a PhD in marine science, you probably studied a science in undergrad. If your science transcript is less than stellar, maybe you could use a GPA bump before applying to that PhD program. If you are switching fields entirely, and have very little science background, you could use a master’s degree to prove you are capable of graduate science study. Or if you’re straight out of undergrad but have no research experience, you can get great practical experience in the field, lab, and academia by completing a master’s degree. The list goes on, but if you really want to attend a top institution for your PhD, a master’s could be the resume builder you need.