We love the North Face BaseCamp duffles. They are tough, waterproof and have backpack straps. (Waterproof as in it can rain on them, not waterproof like take them into the ocean.) They come in many fun colors. The small and medium have been successfully carried on United and Southwest. Lufthansa said the medium was too big for a carryon. They are kind of strict. I think they'd be good for carrying towels and stuff on the dinghy.
http://amzn.to/17h8p9M

We also carry a couple of very lightweight, cheaper dry bags for electronics that we always take to the BVI.
http://amzn.to/1CAshNI