Does the OP want a single, large bag that they can slide their entire backpack inside of to truly swim? Or a drybag big enough to use as a liner inside the full body of their backpack? Or individual smaller drybags, like one one for your sleeping bag, one for your clothes, etc?
The crazy-superlight bags are waterproof, but tear absurdly easily. I have even ripped them just gently stuffing a puffy coat into one. An example of these are the Sea to Summit "Ultra-Sil Nano" bags. Avoid these at all cost. The Sea to Summit regular "Ultra-Sil" dry bags are light and will survive normal use with care. The inexpensive Outdoor Products bag that DT mentions work fine and are about the weight fabric that is durable enough but still light enough to hit the sweet spot. OR 'Ultralight' (not truly the thinnest and lightest out there) are also durable enough if you are careful, in my experience. As a long time sea kayaker I don't think you really need to go for the heavy duty drybags. Anything that is seam sealed and isn't so light that it will tear within normal use should do fine. They all work on the same roll-down principle: basically forming a labyrinth seal. With light drybags, you just need to avoid stuffing sharp objects into them (this includes zipper pulls- fold/stuff the object so the zipper is on the inside), don't place the bag on rocks or twigs when you stuff them, and don't pull up hard on the mouth of the bag as you stuff. The stitching at the roll-top closure of bags is where you can get a tear to initiate. Use a slightly larger bag than you absolutely need, stuff it with medium pressure, and then 'degas' the bag to achieve a smaller package.