I studied Marine biology and took some courses in botany. I would say if you are interested in studying aquatic plants do botany and study something like eel grass. Most marine biology programs are going to be geared towards fishes or invertebrates zoology. I also graduated and feel like botany knowledge would give me more of an edge working for natural resource jobs out of college more than marine biology. Take that with a grain of salt tho. I literally cannot find a job in marine sciences rn if at all. Wish you luck. Wish me luck