We purchased a set of 3 Watershed Dry Bags at the Miami Boat show in 2007; a bag for the computer, a large bag for using like a back pack, and a smaller bag for wallet, phone, and other valuable that one wishes to maintain in the working state.
Today (2021) , all of them are still like new and we use them quite a bit. The biggest trick is once you get them sealed, getting them back opened up can be a pain. The key to keeping things dry is two fold: 1) A zip lock like rubber gasket gets squeezed and locked together and 2) the top gets rolled and clipped in a permanently rolled up position.
Getting the bag open is the trick. You need to form the top seal into an "S" shape and then pull. There are pull tabs built into the bags and they work wonderfully. The rubber coating inside the bags are still good 20 years later and I hope to have more years of service out of them.
If I need anymore dry bags they will most certainly be from Watershed.