If zip-lock sandwich baggies didn't leak and were heavy duty enough to last a while under kayaking use, then you wouldn't need these Aloksak bags. But in the real world these bags are the only zip seal bags to use. Aloksak bags come in several sizes and each size is sold in a set of two. The smallest is 6"x6" which is a good size for carrying 3 small areal distress flares, a pocket air horn, electric car keys, or a small hand held GPS (i.e. any of the Garmin E-trek series). So there's enough uses for at least three bags that size. The next bigger size is 9"x6" which is good for protecting larger hand held GPS units, or small roll of TP and you can use a couple others this size to pack out used TP. The largest size is 13"x11" which is good for protecting books, note paper, small charts, or a VHF radio.

Aloksak bags come in a pack of two.

First dimension is the width of the opening so a 9" x 6" has a 9" ziplock opening. A 6" x 9" has a 6" ziplock opening (this one is best for smart phones).