If you are at all concerned with having time to be with your future baby, the Navy is not the place for you.
Sure, you can manage to get a short hiatus from being deployed, but you will still likely be at sea for months and months at a time with an 18 month at home, and a 4 year old at home who knows just enough to know that mommy has left her. If you want great maternity leave, don't join the military. In fact, might I suggest Canada for that?
As for marriage, I am very firm believer in the notion that no one should get married unless and until they feel absolutely compelled to do so. Yes, you'd get a few extra dollars, healthcare, and the right for your husband to shop at the commisary. That really isn't much, and it certainly isn't enough to serve as motivation to enter into a legally binding contract that you are lukewarm -at best- about.
And as an FYI, I am a happily married Navy spouse, so it isn't like I am just blindly anti-marriage.