six-what is the deal with all the different lengths?....what is the differenec in using them?.....also can one person use a two person like the one i linked?
Shorter lengths turn faster, but generally go straight slower. A long, tapered design like you will see on ocean kayaks can get going pretty fast, but you will not be doing any quick maneuvering in it. Plus, you can't put as much crap in a short one, if they will be using it for overnight trips (or lots of beer).
The short, squatty designs are for fast moving rivers/whitewater. The shorter, wider flatwater kayaks (~12-14') are good for slow rivers and lakes and are going to be more comfortable and probably easier to use for novices. The longer, narrow kayaks (16-17') are generally for oceans and take some getting used to since they are harder to maneuver and you have to sit inside the kayak, usually with spray skirts and all that fun stuff. Those aren't hard and fast rules, but that is what you will usually see people using them for. Those lengths are for single person kayaks, by the way, so add a few feet for tandems.
I personally do not like sit on tops because when it rains, you and the inside of your boat get wet. Think of being in a canoe during a downpour. With sit inside models, you can wear a spray skit, wet top, etc. to keep you and your boat dry. For day outings, it does not matter because you can just turn around and get out, but on multi-day outings, being wet in a wet boat for several days gets miserable really quick.
Same as a canoe, one person can use a two person model just fine. It will be harder to maneuver because of the extra length, but one boat is cheaper than two...