Seconding Sarah L and delurked -- unless you have a slim straight figure in the bust area, you'll want something underneath for support. Also, bonnie's idea is genius.
I find that rashguards are great in theory. I dislike the sticky feel of sunscreen and the way it affects my sensitive skin, so the temptation to just pull on a long-sleeved garment for protection is strong. And then, reality sets in. Reality: out of the water, these are made of non-breathable synthetics and can feel just awful in hot, humid weather. In the water, they make me cold and want to scream (because they do feel like you're wrapped in wet fabric, since you are.)
So, there you have it. I love the idea of these but have found them hard to make peace with.