I don't know exactly how big your brest are, but if it is really really flat, like a kid, then i suggest do away with the push up bras and padding. It will make you look really odd and your 'breast' will look hard and unnatural.

Good thing you wanna impress your bf. I know all the yadda yadda about love the person inside not outside bla bla. But you still wanna impress him right? I suggest get a mid-high cut one piece swimsuit. You'll look sexy even without big breast. and make sure it doesn't come with super hard paddings, cause it'll make you look like you have metal breast.