You can be modest in a short dress with no sleeves and a plunging neckline. I know, because I did it while on a Caribbean cruise last summer.
(Click the thumbnail for a larger image.)The head covering is called a snood (scroll down). I could also have worn a hijab scarf in black, or in any color that matched one of the colors on my dress. I would've bought it in a very light fabric, something gauzy that would let the breeze circulate beneath it so as to make my neck comfortable. Modesty doesn't have to be all about buying a modest dress or suit; it can be about buying an immodest dress and then finding a way to make it modest. Some folks do it by layering, like me. Folks with a bit more sewing skill might buy extra fabric and sew on some sleeves, or a ruffle at the hem, or cover a skirt slit with a bit of a kick-pleat.
Layering doesn't have to be a hardship, either. The temperature on this cruise never got below about 87 Fahrenheit at night, and the humidity was never less than about 90%, yet I'm pretty sure I was just as comfortable as everyone who was letting their skin show (and getting sunburns and skin cancer). Note the VERY THIN, breathable, breezy black cotton T-shirt from The Gap, and the VERY THIN, breathable, breezy black yoga pants beneath.
Also note the weird expression on my face. A bug had just flown up my nose.
