The final decision on this will rest with your biology teacher and/or your guidance counselor. It will depend on how the marine biology class is taught; if it is an upper-level class (for 11th and 12th grade), then the instructor may make the assumption that students already have mastered certain concepts. If you are the only one in the class who has not taken biology, you will be at a serious disadvantage.
As far as taking marine bio instead of biology, I would tell my own students (I'm a science teacher) that that would not be a good idea. There are some topics that you need to learn about that are probably not covered in marine biology, such as genetics. If you enjoy science classes, then take both (biology first). If you only want to take one of these classes, it should probably be the regular biology class.