You can get lightweight sandals designed for wet wading, but I would second the warnings above against trying to do it when the water is just too cold. You might be wiser to wear hiking socks and lightweight fleece pants under a pair of nylon stockingfoot hippers. You could still wear wading sandals over those and keep the overall weight/bulk down for backpacking. Fleece pants are light and compressible, hippers are a lot lighter and smaller than a full pair of waders, and sandals are lighter and smaller than boots (and absorb less water).