Junior/Senior year of engineering school is hard to juggle with anything. That said, it is not impossible and though it may sound cheesy, is "character building" IMHO. I am just finishing an 18 credit term (of very challenging senior year engineering classes) while also holding a senior billet in my Battalion. It has probably been the most difficult term of my college career, but it IS doable. Being organized helps, and learning to prioritize is the real key.
I will say that in the past I had felt engineering involved about the same amount of work and difficulty as a liberal arts degree, just in different ways. Now I would venture to say that it IS a lot harder. My point being, do engineering because YOU want to, not because you think the Navy wants you to. It will not be a walk in the park, and all the Navy really cares about is whether or not you get a degree. So choose your major wisely.