The SA System Max series boxes use a gasket and vacuum sealing to make the box waterproof. If you simply close the box it is not waterproof. You need to push on the center of each side of the box to make a airtight vacuum seal with the gasket. If you do this, the box is waterproof. I have dunked a box not doing this and some moisture does come in. If you make a vacuum it is very water tight (even airtight). Overall, I have been very happy with these boxes.

Chris