Due to the environment I live in I wanted to get a new duffle bag and backpack that were waterproof. What I found was the 105 liter Shackelton duffle bag (about $90) and 30 liter backpack (about $60) by Gage.
The duffle bag is fantastic since the size will allow you to practically carry a small body around and it has backpack straps you can pull out if you have to walk very far. Even if you don’t live on a boat think how useful this bag can be when you are watching your luggage sit next to an airplane in the rain (no wet undies here!).
The backpack is also large and I carry my laptop to shore with it. Then I can go shopping for boat parts and simply stuff them into the bag.
Both seal by rolling the opening down and then strapping it into place. If you want you can check out this video the company makes about the duffle bag.