My partner has one of the shock absorber swimwear sports bras that she wears under her 2xu trisuit (which doesn't have any support). The bra is designed for swimming in, so dries quickly, has no wires and is chlorine resistant. She's a 32DD and really happy with combo, everything stays put.

http://www.shockabsorber.co.uk/swim/bikini.html

They don't seem to be available outside the EU, however this site ships internationally (it's where my partner ordered from):
http://www.figleaves.com/uk/nsf/qwis...bsorber%20swim

Reviews:
http://www.lessbounce.com/acatalog/S...ikini_Top.html
http://reviews.wiggle.co.uk/7867/536...ws/reviews.htm
http://reviews.wiggle.co.uk/7867-en_...ws/reviews.htm