Kayaks come in all shapes and sizes and it's important to choose the right one because not only it will let you get the most enjoyment out of your time on the water but having the right kayak can also impact your safety on the water. That being said, narrowing down your options is actually pretty easy and it starts with identifying how and where you'll be using your kayak.
Will you be paddling on small sheltered bodies of water like ponds or small lakes? Or will you be taking the kayak to big bodies of water that are exposed to wind and waves? What's the water temperature going to be like most of the time? And when it comes to paddling, is it more important to have a kayak that's fast and responsive or are you more concerned about having a very stable kayak that would be difficult to tip over?
With these questions in mind, your first and biggest decision is whether to go for a sit-on-top or a sit-inside kayak and there are pros and cons to both.
Sit-on-tops (SOT's) are the most user friendly. They are very stable, easy to get in and out of and there is no feeling of confinement on them. They're also self-bailing which means that water drains through small holes called scupper holes that go right through the kayak. Another great thing about sit-on-tops is that you can slip on and off of them very easily which makes them a great choice for kids to play with.
All these features makes sit-on-top kayaks ideal for more cautious paddlers, for warm environments, for rougher water conditions, and for people who are paddling the kids who love to swim. The downside to sit-on-top kayaks is that you are guaranteed to get wet while paddling, while sit-inside kayaks allow you to stay dry.
Sit-inside kayaks are great for paddlers who will be in cooler water, who want to stay dry while paddling and who want a faster moving kayak. The only downside to sit-inside kayaks is that if you flip for some reason, recovering isn't a simple process because your kayak's going to swamp. Once you've decided on whether to go for a sit-on-top or sit-inside, you'll need to decide on the length for your kayak.
As a general rule, the longer and narrower a kayak is, the faster it will travel and the wider the kayak is the more stable it will be but the slower it will be. Most sit-on-top kayaks are considered recreational kayaks because they tend to be wide and ultra stable while sit-inside kayaks tend to vary a lot more in shape and size.
In fact, sit-inside kayaks can be broken in to three distinct categories. You've got recreational sit-inside kayaks which are wider, shorter and have large cockpits that don't feel confining. Rec kayaks are usually 9 to 12 feet long. On the other end of the spectrum, you've got sea kayaks which are designed to be fast and to deal with any type of condition. Sea kayaks are long and narrow and they have small cockpits that are designed to be used with the skirt to keep the water out. A small cockpit also lets you use thigh hooks to grip the boat with your legs which gives you a lot more control over the kayak.
The trade off is that sea kayaks are lot less stable than rec kayaks and people can find the small cockpits confining even though if you flip it's very easy to get out of them. Now as a general rule, it's a good idea if you're going to paddle a sea kayak to take a sea kayaking course because among other things, it teaches you how to deal with a capsize.
In between rec kayaks and sea kayaks, you have a class of kayaks commonly referred to as touring kayaks. Touring kayaks are usually between 12 and 15 feet long and they offer a great compromise of performance, comfort, and stability. Once you've decided what type of kayak suits your needs, you've dramatically reduced the options and most of your decisions at this point impact your comfort in the kayak and relate to features of convenience for the type of paddling that your going to do.