Not having a sufficient Navy to defned yourself is something that the AI will notice quickly and will result in your annihiliation-they actually do watch for this kind of weakness, and you can always judge where you are compared to others via the civilization comparison screens.
I never get too far behind the top dog militarily. Especially if I have a different alignment than them, both bad alignment matchups + huge gap in military mean you are gonna get DOW'd and murdered.
