Hi JM - there is a foam gasket on the back of the door. It compresses when you lock the box and does a nice job of keeping things dry.
Regarding waterproofness, these boxes have done very well in most conditions but will probably allow some water through when riding continuously in "monsoon-esque" rainstorms, or if submerged. My recommendation is a large storage bag Ziploc for anything that has to stay completely dry (electronics, etc.). I'm also going to connect with Storm Industries about the possibility for Cordura liner bags for the raidboxes. They are an ADVrider inmate and vendor and their link can be found here
Any other questions, please let me know.