Large Dry Storage Bag by Dry Pak
Laptop Bag / Fits 17" laptop Laptops
Dry Pak's storage bags are no frill, no padding pure waterproof devotion. Nothing fancy here just a quality waterproof sack you can put your existing laptop sleeve in when you're going on the boat or out in the rain. Combo this with a Trager Transporter and you're ready for anything...except a big fire maybe, that wouldn't be good.
We've done the measurements and most people with laptops will probably want to go with the large size. The medium would work with smaller laptops and the small would be good for random electronics and equipment. Even with this gear there will be a lot of room left over for clothing and whatnot.