Seakayaking puts a waterproof case to the ultimate test. Cases that never leaked using them while hiking or on the bike (and even on the motorbike - speeding down the German Highways in non-stop rain on "naked bike" is an exposed session for a map case) don't guarantee the needed protection to the elements on the deck of the seakayak or in the pocket of the PFD.
Roll closure systems with Velcro are spray-waterproof and do out-stand a lot of rain, but don't offer protection against submersion -useless on a kayak deck. Roll closure systems with a buckle perform better, depending on the stiffness of the fabric - and are a good solution for dry bags. The problem with smaller pieces like map cases and little pouches is that there isn't enough room and resistance to close them properly.
The simple principle of squeezing the plastic fabric of the pouches together, proved to be best way to create a hermetic seal. The clip of Aquapac does a good job (of course it's important is to keep it clean of any sand!). Very annoying with the Aquaclip system is it's tendency to get separated from the pouch - especially with the small opening of the waterproof case for the VHF-Radio this often happens. The critical point of all waterproof cases are the edges - when they tear in (as shown on this picture) it's over with waterproofness.
Another proven system is the Zölzer-system - shown above is the recent edition where you don't need to disassemble the complete rail to open the case: you can slide the case in and out. I do prefer however the simple old system, where the rails is bolted through the fabric of the case. This takes more work to open and close the bag - but the edges of the case in the new system are (to) fragile for the sliding operation. Although the new case was always used with care: within a year the edges split up. The old case of Zölzer is more robust: the edges of the case are still intact after 8 years of intensive use. Minor points of the Zölzer-design are the unwieldy character of the aluminium rail and the little pieces - I have dropped several screws in the water...
The best waterproof case? Home made cases like Elko, Axel, Paul and Reinout make... So satisfying that "Do it yourself " performs better than money can buy! ;-)