broadreach and absaav,
First I'm not an expert on kayak diving - these were the first really successful ones. OTOH they were the first ones with a suitable kayak. I got tired of trying to make my whitewater stuff do. I tried my inflatable rafts, inflatable kayaks, and finally a whitewater sit-on-top with very generous outriggers. All the options carried the load well and were stable at the dive site. However, due to either the high profile in any wind (inflatables) or sitting deeper in the water than designed (outrigged hardshell ww sit-on-top) they were just too much work to get to the dive site.
So this last weekend I rented a Malibu 2 XL from Monterey Bay Kayaks (they had a used kayak sale going on) and tried it out. Our first dives were off of Del Monte beach behind the shop. We both took 2 tanks each in our separate kayaks (my buddy has his own single - think it is a Cobra Dive and Fish). It carried my 2 tanks very well. It wasn't fitted with hatches, but they can (and are being) fitted with some. I didn't swap tanks, just did my single backmount with a AL 80 sidemounted to my left. My buddy did swap tanks on his kayak without issues. As a result of this test a big giant (13.5') Sea Lemon nudibrach attached itself to the top of my car and wouldn't come off until I got home (I bought their last used one).
The second dive we tried using just the Malibu 2 XL and carrying only one tank each. He is 5'11'' and 180# and I am 5'6'' and 140#. It was a tight fit with him in the back seat. It probably would have been better with the shorter person in back. Paddled well and was a stable and adequately roomy platform. This is where hatches to store gear would have been really nice. So much so that I have ordered the big oval hatch and one smaller round hatch.
We did use an anchor system, a 5# (medium one at West Marine) folding grapple anchor on a 3' length of chain and a 2:1 scope on the rope (paracord).
The biggest issue is loading and unloading the kayak from the roof of my Chrysler Town & Country minivan. Not bad with 2 people. I do need to be able to do it myself. This I'm working on and think I have it solved, but will wait to post until I'm sure.
2 tanks each needs 2 boats. 1 tank each a single kayak works very well and is much easier to get to-from the water. Hope that helps.
PS. If you want to try it out PM me and maybe we can go diving.