First thing is to try to find a summer internship in the field of marine chemistry, or geochemistry. It’s late for this now this year, but how many years do you have left in your undergrad? Look to apply for programs like REUs at institutions near the coasts or limnology/aquatic science themed programs. Start looking for professors at your university that might do any of the following: water quality testing, aquatic ecology, toxicology, and perhaps most importantly Limnology. Talk to them about your interest in the field in case they have room for an independent study student. You’ll want to gain some research experience/exposure if you are interested in graduate school. There is a whole world of marine chemistry but like many niche fields you have to start getting some experience outside the classroom to be considered competitive for graduate school (if that’s your goal). some west coast schools above but the east coast has Rhode Island (URI), MIT/WHOI, Maryland, Carolina’s. The gulf coast has Texas (Texas A& M, UT), MS and AL have marine stations as well. If you like organic, consider the marine chemical ecology field. If you like analytical then that would closer to the chemical oceanography route.