20 and 21 and both facing 4 years of sea duty on different ships? The odds of your success are not good, hell, not even as good as when only one spouse is in the Navy.
Both of you are at the beginning of a professional career that you may or may not follow at the end of your enlistment. Neither of you know what lies ahead. School they say is difficult, and it is only because you are new to the field. Once in the fleet, it starts all over again, but this time on plants that are on ships that go to sea. Not just an experience, but a lifestyle that will go on for four years. At this point in your enlistment, you have no idea of what that world will be like.
My advice is to stay engaged and do what you can on duty station assignments. It's a lot easier to get married later then to get unmarried later, which is more likely then not to happen before your contract is up.
I have seen people who miss not being with loved ones, and it is difficult. I have also seen people who are in the process of a marriage breaking up and are 5,000 miles away and can do nothing to stop it. The latter tears people up.
People here are trying to tell you the facts of life as they have observed them, or in my case lived them.
Married or not, the time you think you two will have together in the fleet, based on your current experience, isn't even close.
In closing, base your decision on the real prospect that the two of you will only see each other once or twice a year, and then only if leave time can be coordinated.
That's a lot of time apart.
Good luck to you in your career and life.