Regardless of the material a kayak is built from, additional floatation is mandatory. You can use bulkheads with hatches, float bags or both. A sea sock is also an option, but should not be used without float bags or bulkheads.

The critical thing is to limit the amount of water that can enter your kayak. As you learned with the canoe, a boat that is completely swamped is all but useless, unless you have other paddlers that can assist you in emptying it. Even if you could get back in, the chances of being able to pump/bail it out without capsizing again are nil. With proper floatation in the bow and stern (both are required), only the cockpit can fill with water. The boat will float high enough and contain a small enough amount of water to allow you successfully empty most of it by yourself while swimming, assuming that you're wearing a PFD (as you should be). This is done by rolling the boat over, then lifting the bow slightly to allow the water to drain. Attempting to pump out a boat while swimming is a waste of time and energy. You're best bet is to get back in, even if you cannot fully drain the cockpit. Once you're back in the boat, you can install your spray skirt to pervent any additional water from entering, then pump out the remainder of the water and be on your way again.

It sounds like you're not familiar with paddle float rescues or assisted rescues. If you don't already own them, buy a pump, paddle float and spray skirt. Do some reading about self rescue and practice wet exits and rescues in shallow water until you're confident that you can get back into your boat. If you have clubs or instructors near you, contact them to find out if they run skills sessions. Practice self rescues and assisted rescues until you can do them without even thinking. The goal is to get out of the water and back into the boat as soon as possible. I cannot emphasize the importance of this enough; it could literally save your life.

Learn to eskimo roll. There are two reasons for this. First, it's much easier and safer to roll back up in the event of a capsize than it is to wet exit and re-enter. Secondly, having the ability to roll allows you the freedom to work on bracing, sculling and other advanced paddle strokes without to worry about wet exits and re-entries if you make a mistake and capsize. Your paddling will take on a whole new dimension, once you learn how to roll.

One last suggestion: read the book "Sea Kayaker Deep Trouble". In my opinion, this is required reading for every paddler.