Most of the physical oceanography students I know studied mechanical engineering, physics or mathematics in undergrad. Strong physics and mathematical skills are very important. Programming is also a plus. Having a few earth sciences courses is good, but the physics and math is much more important, so I think you will be in good shape.
However, beyond the basic background, clear ability to do research is very important in PhD admissions. Even if the applicant doesn't have any research experience, there are ways he/she can show that potential. A clear statement of purpose that outlines and contextualizes the problems the student wants to address in graduate school will catch the eye of any professor looking for students. There many students who have posters at AGU, GSA ect, however, the truth of the matter is that most undergraduates with research experience entering graduate school aren't ready to hit the ground running anyway. Professors know that, as they have had students before.
For courses, I would say skip the environmental modeling course unless it is directly related to what you want to research. Take the remote sensing, and instead of environmental modeling, take a full on numerical modeling course. A rigorous undergraduate level (or if you can stomach it, graduate level) will give you a very strong foundation for whatever simulations you may or may not to run during your career as scientist, even if you start out as a purely observational scientist. Fluid Mechanics is also a no brainer if you still have time.
Finally, the oceanography researchers at LDEO (columbia university) are top notch. So are the ones at University of Washington, Hawaii, and Rhode Island.
Good luck on your applications, you have a lot of time to decide what you want to do (since your junior year just started I'm assuming).
The best advice I can give you is look at what researchers are doing on their websites and try and read a few of their papers. If you know any professors with these same interests, ask them what you don't understand. Earth Sciences is much different than math (I came from pure physics), spend some time trying understand what an oceanographer does. If you can figure that out, you will be more than prepared to reflect that in your statement of purpose and you will be a stellar candidate for graduate school.