What is the difference between a B.A.S. and a B.S.?

The difference is in the coursework that you do. It will vary by the universities, so just take a look at the coursework required for the degree and see if its courses you would like to take. While I don't know this to be true or not (I have a BS), I'm told that BAS generally don't prepare you for grad school as well as a BS does. Please note, this answer is specific to America and your mileage may vary in other countries.

As far as what degree you should get, I recommend that you not focus solely on marine biology. Even though I do have a BS in Marine Biology, in some ways, it did stunt the courses that I took. When I met with my advisor and stated that my goals were to get into grad school, he charted a course that looked a lot more like an ecology major. For you, I would recommend that you either get a degree in the sciences, e.g. biology/chemistry/physics. It'll give you a lot more options should you decide halfway through it that you don't want to continue on into marine biology, which is a very niche field.

For the most part, what most people think of as marine biology requires a grad degree, either a masters or PhD. It tends to be grunt work, and not so much research, at the bachelor's level. Any of the above science majors will prepare you well for marine biology. Keep in mind, though, that marine biology is a very broad field, so you would also be well served to volunteer in some labs to narrow down your interests.

Science is so awe inspiring - get the credits and immerse yourself in every subject- you'll absolutely love it!

Biological science degree and former Veterinary tech experience as well as science educator experience