1) You will likely get a slightly better deal from an online travel agent that specializes in cruises. Some lines have rules that prevent agents from selling below the line's direct pricing, but the agent can kick back part of the commission in the form of shipboard credit or other perks. You may save a couple hundred dollars this way. There are pros and cons to using an agent. Either go with a large agency that seems well established or book directly.
2) If you want an exterior cabin near the center of the ship, you can't play chicken with the pricing. Book now if any of these cabins are even left at this late date. If you want the lowest price, and are willing to take whatever cabin is available, then you can wait and see, but if the ship is nearly full then prices will go up rather than down and it may sell out. FWIW, for this particular sailing, I personally would watch the price until the end of July, then book an inside guarantee. (No idea if that's the best strategy on this exact route, just my gut feel.) I think you should have a good chance of getting a cheap upsell offer to a balcony or ocean view, but even if you don't, there's just not that much to see between Charleston and Bermuda that can't be seen equally comfortably from a deck chair. I'd rather save the money and sleep in an inside cabin.
3) Carnival is not that formal. For dinners, you would be following the dress code if you wore long pants and something like a polo or Hawaiian shirt. There will be some men who show up in shorts and sandals anyway. They will probably have one formal night, but participation is optional. Specialty restaurants may enforce the dress code more.
