Most Helpful Guy
I believe you can purchase skin colored silicone pads that aren't large enough to give off a stuffed bikini look, but enough to keep your nipple and areolacovered. If you do not wish to do that, I would recommend trying your two piece well, even double knot or using little clips to hold the fabric tight to your body. If all else fails, a two piece will pretty much guarantee you piece of mind.
Most Helpful Girl
Don't have boobs above a B-cup, buy a swim suit that fits, or find one of those nipple covers or pasties.