: Anyway those of you old enough to remember Magnum PI, at the close of a
: show or when he was stressed he would frequently get his
: "kayak?" and take it out. Is that a kayak? It seemed much more
: narrow than a kayak and he was sitting on top of it instead of inside it.
: What is it designed for? What is the advantage/disadvantage of this type?

The boat Magnum paddled is called a surfski. It is similar to a sit-on-top kayak with the exception you noted - it is narrower. I've never used one (or any sit-on-top), but my understanding is that they are designed for one thing - speed. Current Designs makes a fiberglass version called, appropriately, a "Speedster". It is long and narrow, and looks like it can live up to its name. Some kayak shops allow demos, so if a local shop carries Current Designs kayaks, you might be able to give it a try.

As for building one, you are probably out of luck. I am not aware of any plans or kits for sit-on-top types of boats. There were some messages on this board a while back on modifying plans to make a sit-on-top, but I haven't heard of anyone actually attempting it. The thoughts were to use the kayak's hull design, and then make radical modifications to the deck to create the open cockpit.