I have a bigger bust and bought a tankini top from Very, it sits perfectly in place. I hate showing the tops of my legs so I have a swim skirt from Bon Marche.
I have to admit I feel really good in it which is a bonus, I'll work out more.
I did have a swimdress but it didn't fit well but I think it was the style (moulded cups, too heavy).
M&S used to do a really good swimdress, they're not on their website now but there are lots of new ones on ebay (black with white straps).