I also wore a padded bathing suit, and that seemed to work for me, but you could certainly sew in some additional reinforcement! There are lots of products out there that you can even buy at a fabric store in the undergarment section that are designed to be lining for bras, or tops, etc. You could probably tack some into your swim suit pretty easily, if you are finding that even padded tops aren't providing enough coverage.
I'd also suggest looking online at Lands End. They have really nice, high quality swim suits, with heavier fabrics, and with different options for the tops, i.e. padding, underwire, etc., and the tops can be purchased in bra sizes, as opposed to S,M,L... They are a bit more expensive than say, Walmart, but they are worth it if you can swing it.