To help answer Kenny's question, it depends on the hull configuration, and the weight distribution and other physical properties....
If you were to do a lot of web research, you could read about weight load capacities and other specifications, and make decisions based on your personal preferences.
What I have found is that how you choose to transport your kayak to and from where you come and go to is almost a more important decision then anything else. Whichever kayak you choose, put as much energy into your method of transporting it, lifting it on and off your vehicle and ketting it into the water. Nothing worse then having the damn thing come off on the freeway!
I have a Yakima roof rack system with saddles and tie downs, and a kayak dolly to hike the kayak to the beach.
If cost is an issue for you with just the hull selection only, you might miss out on recognizing some of the other gear costs.
To pick out just the right hull for yourself, you might want to find a way to paddle different ones. Maybe there are rentals your could try, or paddle clubs near by, or friends near by...
Newer models are very different in design then older ones though, so do both a paddle test and a web search for info if you choose to buy a brand new one.
Sit-on top kayaks are not specifically designed just for SCUBA diving though, they are mostly designed for other things like fishing, but they have enough features for diving to get you by.