I just ran into the Bra Crusader's blog article about her intention to participate in a triathlon with her husband and the difficulties of being a G cup.

The recommendation she found for the swimming problem is to just swim in the sports bra, which may damage the bra.

My recommendation, buy a bra specially for the event. In eBay.co.uk you can find many Shockabsorber bras for up to 1/10 of the price. The issue remain on being able to try it on .

Just keep in mind that Shockabsorber bras run really tight in band and cups so you might want to go a couple of sizes bigger in the band and a cup larger.

Follow The Bra Crusader blog, it's got great bra information