Before you buy a sit-on-top kayak (or any kayak, for that matter), you should take stock of where and how you plan to do most of your paddling. Most kayaks can be used for a variety of activities, but understand that no one kayak excels at every activity. Choosing a boat means mulling over a long list of characteristics and compromises. Be wary of anyone who tells you about the "best" kayak overall, there is none. There are, however, a few guidelines to help you decide which boat is best for you:
Length, Width & Speed
Length and width affect speed and maneuverability. When it comes to speed, it's all about surface area. A long, thin line allows paddlers to slice through the water quickly which is a real advantage if you plan on touring, but a drawback on twisty rivers. A 17 foot-long kayak will go much faster than a 9 foot-long kayak, but a 9 foot-long kayak will turn easier than a 17 foot-long kayak. A narrow kayak is faster than a wide one, but wide kayaks tend to be more stable (depending on hull shape).
Hull Shape affects your kayak’s performance. Depending somewhat on width, flat or smooth-bottomed kayaks (U-shaped) have more secondary stability, while keeled kayaks (V-shaped) have more primary stability. U-shapes may feel tippier at first, but stay stable in moving water (rivers, surf, etc.) while V-shapes feel most stable in flat water. The tri-form hull of most sit-on-tops combines both primary and secondary stability with a long center keel to keep you going straight, and two "shoulders" that act like sponsons for secondary stability.This tri-form hull generally sacrifices a little speed, but adds a lot of stability (which is great for cross-over sports like fishing or diving).
There are lots of different hull shapes, but basically V-shapes encourage a boat to go straight (good for touring), and smooth bottoms encourage a boat to spin (good for surfing, kayak polo, or river running). Whether or not a kayak goes straight is referred to as "tracking." You'll want a kayak with good tracking to cover distance on flat water, but you'll want less in whitewater. Chances are, for recreational paddling, you'll want a kayak with a keel (some kind of V-shape on the bottom), so you can travel more efficiently. If you expect to spend equal time on flat and moving water, consider buying a short kayak with a keel (it's all a continuum, remember).
"Rocker" is another term used to describe hull shape. Think of the keel as something you'd notice on a kayak cut in half. Rocker is most noticeable on a kayak cut cross-ways. A kayak with a lot of rocker would look like a "U" in a cross-ways view. A kayak with little rocker would look like a line. Keeping in mind the idea of surface area and water displacement, you'll want a lot of rocker (one round point touching the water) for maneuverability, and you'll want very little rocker (a long, thin line of points touching the water) for touring. For recreational paddling, pick something in between.
Single or Tandem
Do you plan to paddle solo or tandem? This is one of the most basic questions you'll have to answer. While one person can paddle a tandem alone, it requires sitting in the rear of the kayak while ballasting the front. The kayak will move, but not at its optimal level. On the other hand, it's a lot of fun to go out with a partner, often safer, and usually cheaper than buying two boats. There are a few tandem sit-on-tops that have a jump seat between the front and rear seat wells. This seat arrangement makes it possible to balance weight for better performance when paddling alone, and may be a good option to try if you want to paddle both tandem and solo.
Your body determines how the kayak will perform. You probably wouldn't buy new pants without trying them on first. The same rule applies to kayaks. When you test paddle, you aren't so much looking for mechanical failure as you are trying to get sense of how the kayak fits. It goes beyond just height and weight-people carry weight and proportions in different ways, and these differences translate into how you balance in a kayak. You can always learn how to work with different kinds of kayaks-experts tend to balance better than beginners-but know that it will take time and practice, especially if you decide on a specialized kayak.
There are a few other things to think about when choosing a kayak. Where will you store it? Can your garage or basement hold a 16 foot-long kayak? How will you transport it? Do you have roof racks or would you rather put the kayak in a truck bed? Even color choice is important. Do you want to be seen, or would you rather be camouflaged? Considering these issues early on will ensure you enjoy your kayak for many years.