Here's a shameless review of a field-tested product from the Wisteria Shop at the Martin House Complex:
Those cloth-like, fold-able green bags featuring the stylish Martin House birdhouse graphic are for more than just groceries or paperwork (Mary Roberts, take note); they're great for toting all manner of summertime and beach accoutrements. Take it from me - I used one of the bags recently on my Spring Break trip to Mexico. They fold up into a compact package that's easily tucked into a small suitcase, and they're lightweight but tough enough to hold your towels, sunscreen, Frisbee...even that trashy paperback. And, after you've accumulated a few treasures on vacation - be it seashells or a hand-painted coconut lamp - you can use the tote as a durable and flexible carry-on for the return trip.
No, it doesn't double as a flotation device, but I think that the Wisteria Shop R&D Department is working on that.