Since you know that you want to work in the sciences, the best thing to do is to find a local aquarium, zoo, or science center and volunteer in various areas so that you can learn about the different things that interest you. Taking a variety of classes while completing your diploma or undergrad prerequisites is also a good way to explore a variety of topics as well.
Internships are usually geared towards people who have an idea of what they are interested in as they can be intensive. Determining the best place for an internship would depend on what you are interested in studying. There are a wide variety of internships for biology and marine biology - they are also all over the world, so you can meet new people, see new places, and learn about science.