Another option is an airtight backpack like those used by packrafters. Good for waterproof in a rainstorm but also good as improvised life preserver if it has an air purge/inflation valve like Watershed dry bag backpacks (if you're willing to pay more you can get them in black btw) have as an option.
You do have to deal with the flipside of having an airtight backpack though which is moisture management on the inside, if you put anything that condensates like a cold drink or has moisture like dirty gym clothes you'll need to make sure to keep the top opening opened to air out. Attaching a mesh bag to the outside though provides a mechanism for drying out items that retain moisture.
Beyond Watershed, another decent option is Patagonia Guidewater, Fishpond Thunderhead, or Simms Dry Creek. Have heard of waterproofness failures though with any given zipper based airtight bag whereas haven't really heard of any with Watershed's ziplock style closure. You don't get any internal organization or padding though. Though I've found that throwing a picnic blanket and spare jacket / insulation layer at the bottom provides some padding for a laptop and has a dual use.