If you are looking for economic options you might also be interested in Lomo gear, see below for link, as they do lightweight and shoulderstrapped versions. The see-through window is a good idea (on both Andrew's and my links). You can get very much cheaper options (e.g. Wilco), but they are usually heavy-duty PVC and difficult to fold in cold conditions. The plus point of heavier-duty versions is that they (usually) protect the contents better if they are "thrown in the boat"!