Sports Illustrated model May Andersen got into the Cannes Film Festival spirit ... and took her left breast out for a little sun while on a boat trip on Thursday. It's all about showing your cans in Cannes!

While her shipmates -- babe magnets Kid Rock and James Blunt -- foolishly took in the harbor view, May's wardrobe malfunction went mostly unnoticed. Somebody send these rock boobs a nip slip alert!

Luckily, there were no flight attendants in the air rage-licious model's vicinity, and authorities were not called.