I found a kayak a perfect partner to diving, better than a cumbersome stinky noisy motor boat. Cheap to run too!
They are many out there: easier to use a sit on the top, but nothing wrong to use a normal sea kayak... need more technique but they have they advantages.
You can use a wide kayak, stable and in some of them you can even take the dog and fit an engine... but they are slow and heavy. the with interferes with the paddling tech and they are heavy to pull. On wind conditions you drift hopeless: you should never get out a bay with this thinks!
A narrow kayak is more unstable and need some learning. But pass this point you move fast with a nice and economic stroke. Wind becomes easier to manage and your range increases a lot.
On the middle, one of the most comun is the scuper pro, a good, reliable and stable kayak that have proven over time to be a good value, and a cheap second hand. The cobra tourer is a good value too.
On my case I opt for something fast, real fast (the faster is your hull the less effort needed for paddling) and I can get almost anywhere, make crossings to islands and other weird stuff. Is an Cobra expedition with 3 hatches, soft sit... He still performs OK with lots of load: +40kg fish and diving stuff. I've been taking wind on the face until 25 knot wind + 2 m waves and still going.
My advice is to rent a few before buy anything. For a few bucks you can get them for a day before make the right choice based on your experience.
That will get you started:
http://www.sit-on-topkayaking.com/
http://www.sit-on-topkayaking.com/KayakData/Reviews/KayakReviews.html