I like the idea to go with a vintage style swimsuit. Personally, I feel that they are more flattering in some ways, and if it makes you feel more comfortable, then go for it!
Also, I like to wear a sarong at the beach, and I'm sure you could find something like a skirt to wear into the water. If you feel uncomfortable with your body, that's something you have to learn to work through. If it's your boyfriend you are worried about though, you can't control his thoughts and feelings, but you can wear a one piece which covers a little more to make yourself feel better. The imagination is a powerful thing, so it may not matter how much you have on to you boyfriend...
If you want to wear a bikini, wear a bikini! I often feel the same about my body, as I am not fat, but I am not fit/toned, and have a bit of a belly. When I feel bad about my body, I wear my flattering one piece. But sometimes, I say whatever and just let it all hang out! Who cares what others think? Yours are the only thoughts you can control.