I'm with Jay. I make a lot of stuff with neoprene. I used to use the "Seal Cement" by McNett, but it goes bad really quickly, and it's basically contact cement with rubber dissolved in it. So now I just use DAP Contact Cement you can buy at a hardware store or home center, with spectacular results. Reinforcing with some blind stitching or seam tape (Melco tape) will keep it happy for a long time. I don't have a walking foot for my sewing machine, and I don't like using seam tape, so I do blind stitching by hand. If you sew a zig-zag with a regular machine it will stretch all right, but it will also leak where the needle poked holes in it.

For nylon seams that are exposed to the "outside" and need to be sealed I use Aquaseal thinned with Cotol, or in some cases Lexel.

Good luck!

Mike