Be really impressive, be one of: The Few, The Proud, The Marines. 
My background is I grew up as a military brat, went to college, completed Navy OCS (1982), served 4 years active duty and 8 years Drilling Reserves (damn “peace dividend” drawdown/cutbacks in the 1990s) .
I’m not sure what advice or guidance that you are asking. I agree that a Naval/military career is an impressive addition to a politician’s resume, I would consider a Naval/military career an impressive addition to anybody’s resume.
That said, I’m curious as to how you would parlay a Naval career into a political success.
Which area of politics do you want to enter: local, state, national? The Navy will take you away from the community you want to represent during your 20 year career, making it difficult to develop the contacts in the political community you will need when you do run for office (so it seems to me). If politics is your real goal, I’d think doing a tour or two of active duty, continuing in the Reserves while laying the groundwork to run for office is the way to go.
I agree with XanderCrews, speak to a recruiter and take the tests: the Navy may not want you or you may not want what they are offering. You mentioned being a poli-sci major with an econ minor in an earlier post, so some designator communities are out (Medical, JAG, Chaplain, Aviation ect)(but are possible to transfer to). A truism is “The needs of the Navy come first” (as well as "My recruiter lied to me
)
You might want to be a boat driver, the Navy may be looking for Supply pukes, talk to a recruiter and find out.
Another source of information would be the ROTC office at the college you attend, should they have one there.
Officer Candidate School shouldn’t be difficult for you, earning a BA indicates you are capable of the academics. I had a lowly Social Science/General Studies and they taught me Engineering and Celestial Navigation (with assist from my classmates).
You might not like the military routine -example- at OCS having a well-made rack/bed was important but when I got to my ship, nobody ever checked to see if it was made. My perspective was that “They are paying me, this is what they want me to do, it will get me commissioned, I’ll do it.” It’s a game but there are reasons: it shows you can receive an order, carry it out and pay attention to detail. You give them what they want and they will give they what you want.
As far as being a rabid liberal, I don’t see that as being a negative or positive concerning the Navy/military, at no time was I issued a political philosophy/ideology, what counted was performing the mission/getting the job done.
There are currently-serving members on the Dope that are better-qualified to speak than I but I’ll be happy to relate my experiences/opinions if you wish.