As I am currently working on designing summer things, I've been working with both cotton and bamboo. I wanted to know if items made of bamboo yarn would float. Therefore, I set up the following experiment.
First, I made 3 small (3"x3") swatches: one from size 3 cotton, one from Aunt Lydia's bamboo, and one combining the two. I placed all 3 in a large bowl of tap water. The cotton sank almost immediately, followed by the cotton/bamboo approximately 30 seconds later. Amazingly, the bamboo stayed afloat for over 12 minutes! It didn't sink until the entire surface area was saturated.
Next was the stretch test. I hung all 3 swatches on the clothesline weighted down with a single clothespin and let them dry for 24 hours. There was absolutely no significant change in size with either the cotton or cotton/bamboo swatches. The bamboo, however, grew over a centimeter!
This result backs up the story I heard at the local yarn store. A woman had knitted a skirt out of 100% bamboo specifically for the store's annual fashion show. She wore it for the first time the day of the show, where it started to stretch. At the end of the day, it had lengthened 8 inches! The skirt had stretched so far out of shape that she needed to cut it and put a zipper in to make it wearable at all.
It may add a bit to the weight, but I'm thinking that combining mercurized cotton with bamboo is the way to go for garments. I don't want a crocheted top to end up looking like an over-stretched dress. I think the bamboo would work great for hats and beach things that may end up in the water.