All I could find about the test part on the web site was the schedule, so I only know the names of the test subjects. The ones that came to mind as good for a budding marine biologist were biology, chemistry, ecology (or environment or whatever it was called) and maybe "geography" insofar as it would cover geology, palaentology, oceanography, and planetary science (which, with a name like "geography," may be not at all; I don't think marine biologists need to know the capital of Estonia).
Some trade-offs that come to mind, many involving how long you have to prepare for the test you choose:
1) how well do you know each subject, so how well will you do without studying much?
2) how enthusiastic are you about each subject, since it's a lot easier to learn things you care about, and it might be more useful
3) which subjects would most impress the schools you want to apply to?
I don't really have much of an idea about your academic interests outside modern cephalopod biology in particular, which is a bit, er, more specialized than you can probably expect to encounter. I find that lot of these standardized tests emphasize very "mainstream" parts of the field, like the computer science AP and GRE exams I took had a few general things that everyone should know, but also a lot of historical trivia, outdated techniques, techniques-that-became-outdated, and other claptrap. In very broad, fast-moving fields like biology, this tends to be even worse, particularly since a lot of people study biology to get into medical school, which adds a very anthropocentric bias.
For some areas of higher learning, getting as much familiarity with mathematical reasoning and techniques helps a lot more often than one might expect, and opens doors to a lot of acadamia. Studying neurobiology or biochemistry, for example, have a good deal of calculus sometimes, and a lot of fields need sound knowledge of probability and statistics.
I really wish these standardized tests had a subject of "critical thinking and applying the scientific method," since that's really the foundation someone at the high school level needs to learn to put into practice in any specific field. I hope you're learning a lot of that from these conversations on TONMO, like the discussions of how to interpret fossil cephalopods, or Kat's questions about the spermatophores.
So, I have no idea how to answer your question, but I have a few maybe-useful ideas about how you should go about finding your answer.
I'm sure that whatever you choose will turn out well, in any case. In my experience, tests and the like are usually less important in the long run than actually caring about learning things, which you seem to have covered.