The wonderful thing about kayaking is that it's extremely easy to get started, and the learning curve is very swift. Though you could spend year mastering strokes and understanding water conditions, you don't have to know how to eskimo roll as soon as you get in the boat. Determining which type of kayak - a sit-in or a sit-on-top - to begin with will help you ease into the sport.
Kayaks come in many different designs, but the basic differences between them are whether you sit inside a cockpit, with your legs and hips surrounded by the boat, or whether you simply sit on top of a hull. Most river kayaks - which tend be shorter designs - are sit-ins, and if you're interested in shooting down rivers and over waterfalls, you'll need a cockpit that surrounds and hugs the lower half of your body, which will give you more control over the boat as it careens through currents. But if recreational coastal boating is what you're aiming for, sit-on-tops or sit-ins are both a good choice, depending on your desires. Below are the things you'll want to factor in when making your decision about getting a coastal or sea kayak.
Sit-on-top kayaks, in general, tend to have a wider, flatter hull, which makes them more stable and less likely to flip over (at least in calmer waters). Many are so stable, you can actually stand up on them (carefully). This makes them great for beginners, bringing kids and dogs along, and activities like fishing or photography, where you need to set up equipment, require a broad range of motion, or haul things in and out of the boat while sitting in it.
Maneuverability and skill level
While sit-on-top kayaks are great for beginners because they are stable and easy to cruise on, what you gain from a wide stable base you lose in quick turning power. A longer, thinner, sit-in kayak will respond more quickly and swiftly to paddle strokes, but it will also feel more wobbly. Either type of kayak has designs available that include rudders, but the wider the kayak and the more it draws, the clunkier it will be. Some entry-level kayaks also offer sit-ins with a wide flat base. These can be a great choice if you want the option of wearing a skirt (in cooler weather, say) or if you want to store stuff where it won't get wet. Just remember that if you want to go skirt-free some days, any water that gets inside the boat will make it heavier, and, if you flip, the boat could be difficult to right when it's full of water.
Wide, flat, and slow isn't necessarily a drawback if you're planning to use your boat for expeditions close-to-shore. And, increasingly, sit-on-tops are available in sleeker designs with longer overall boat-lengths.. But if your goal is to cruise in blue or rougher water, you'll need a sit-in touring kayak that's more responsive. And, if you've progressed to a level where maneuverability and advanced paddle strokes are enhancing your experience, you may get frustrated with a slow boat. If you're new to kayaking, remember that learning basic skills goes quickly for most people, so even if you haven't paddled much before, think about how you want to use the boat - you may want to buy a boat slightly beyond your skill level so that you can grow into it.