I was happy to find an inexpensive white rash guard. This fits perfectly.
I ordered a Small
I am 5'2", 32D (if you've never been fitted properly, you would assume B cup...they ain't big), 115lbs and the Small is very fitted. The rouching disguises any lumps and bumps. A Medium would be a looser fit on me and still comfortable, so if you prefer a looser fit, size up. If you have big boobs, definitely size up. The rouching is a nice touch so even 2 sizes up would probably look cute if needed.
This is definitely see through and intended to be worn over a bathing suit, but the dark ones may cover better if you don't want to wear an additional top.
I chose white to help stay cool in the sun. Have not tested that out yet, but it felt breathable.