If you subscribe to Ray Jardines ultralight philosophy – you don't really need these. You should just plough ahead and do stream crossings in your hiking/tennis shoes – get them soaked – and let them dry out over the next few hours.
However, If I know the trail will be mostly dry, except for a few stream crossings – I would rather take a little bit of a hit – and get across the water with dry feet. I've tried sandals, neoprene socks etc – and while not the most stylish option (be prepared to be laughed at), these nylon mesh shoes are a great balance between some sole-protection and minimal weight penalty.
Ignore the sizes. I wear a men's 10 and I can JUST fit into the large size – no way I could get into the medium.
Theresa has the small at 1.5 oz.
I have the large at 2.1 oz.
Available here at Sprint Aquatics, for a whopping $4.95.