A shame you didn't get a photo of the back of the kayak. There are special kayaks made just for diving. Like the one in your photo they are typically rotomolded plastic sit-on tops, and designed for stability and easy reentry.

The ones I've seen have two holes molded in, in the space behind the paddler, to support the diver's air tanks. There should also be some hatches, or mesh nets to hold the other diving paraphernalia: mask, fins, bouyancy compensator, camera, etc. I don't see them in front of the paddler on this shot, so I'm guessing they are behind him, too. Either that or the guy is skin diving, and not SCUBA diving.

Paddling out to a dive site beats swimming out, and the kayak becomes a nice platform for displaying a diving flag, or for holding any fish or lobsters you may catch.

I have not seen plans for these things to be home built. It should be easy enough to modify a standard design. Paul Jacobson