Those Watershed bags look nice. For comparison only, check out the Outdoor Research OR Drycomp Summit sack, 30L size 12 oz and $59 at REI and similar outdoor gear stores. http://www.outdoorresearch.com/site/drycomp_summit_sack.html They operate like most waterproof roll-tops, and have kept my summit gear dry when I've used it (all of 3 times in rain or snow). Small and light enough to pack in a bigger backpack and haul out as a daypack. The shoulder straps are not very large or padded, but your packweight is intended to be relatively small (but crucial to keep dry) so it has not been an issue. OR has a good rep for the gear they make, like other waterproof bags, gaiters, excellent mitts and gloves, outerwear, bivvies and the Seattle Sombrero. "Not intended to be submerged for extended periods of time" which may push you towards the Watershed bags for your adventure lake-crossing a la Grylis.