Plastic doesn’t necessarily equal rec.
There are lots of examples of non-rec boats made of plastic. It's heavier, sure, but the design is what makes the difference between rec and touring, racing, etc.
The flip side, of course, is that being composite or fiberglass also doesn't mean it isn't a rec-boat. WS's "Elite" are made of some kind of composite, are they not?
YoS
Wide boat = inefficient paddling
I agree with Eric Nyre that the main problem with doing fitness paddling in a typical rec boat is that the extra width leads to inefficient paddling and poor form. A narrower boat allows a higher angle, a paddle placement farther forward, and a longer stroke. A wider boat generally leads to a lower angle and a shorter stroke. Torso rotation is inhibited rather than encouraged. So now you are powering the boat with smaller muscles in the arms rather than the large muscles of the torso and legs. Throw in a long heavy paddle and attempt to paddle decent distances regularly at fitness pace and you are likely to have wrist, arm, and shoulder stress injuries, at minimum.
Bingo!
Additionally, wide, stable boats don’t allow you to develop the skills, balance, bracing and rolling skills you need for handling narrower, higher performance boats in a variety of conditions.
Sure. Why not?
Anything works better than sitting on your butt watching football, eating chips, and drinking beer.
Maybe it’s just my height, but I don’t
find that being in a wide boat forces me to go to a wide angle stroke. And while your points all have some validity, I don’t think they make such a strong argument that all rec boats are ruled out for exercise.
Target Heart Rate
If fitness means burning fat efficiently, you have to know your target heart rate and keep within that range, which you would not know in a kayak as opposed to gym equipment that has readouts. If you don’t keep within your range, you can end up burning muscle instead of fat.
Last I heard, you have to workout 20 minutes before you start to burn fat.
In the end, any type of activity will help to keep you fit, but according to what type you do, it will keep you fit in different ways. If you are healthy and can do high activity exercises, that’s your best option.
The only difference I can think of between using a faster kayak as opposed to a slower one, is for the same amount of energy, one will take you further or is it farther. Either way, I think you can get the same workout with either type of kayak, just my opinion.
-Capri
Paddling not efficient for fitness
Even if you are talking about double bladed paddling, which I assume everyone is, I have never found paddling to be an efficient way to increase aerobic fitness, anaerobic fitness or muscle development. You can do a much better and quicker job of all those with weight training and aerobic machines or running. Paddling also limits the muscle groups you can train.
Maybe the story would be different if you are a national class sprint racer and train accordingly.
I paddle for pleasure, even though I used to be able to go pretty fast in my outrigger canoe, sea kayaks and Lotus BJX. But I always trained for paddling season in the gym and on the roads.
Paddling a lot and fast is, of course, helpful to maintain what I would call paddling fitness, which for me is not peak fitness. Doing it in a boat designed for the most efficient hull for any given type of paddling – e.g., double-bladed, single bladed marathon, outrigger – will simultaneously help you develop the best paddling technique for that type of paddling.
Uhhh…
Your heart and lungs can do a lot of "work" or your muscles can do the "work". If you spin the majority of the load goes to heart and lungs. Muscles do the majority if you mash a big gear. The speed of the boat can be equal... same work being done.
I think Tri was on target. He can spin faster in his long lean boat than his short fat one.
(of course one could argue that if he got a paddle with a lower surface area he could 'spin' in his barge)
whats beautiful?
i like mine slender. ever wonder why boats are usually named after women?
I think we all agree, except in terms…
… of the ACTUAL meaning of the wording being used. I’m fussy that way. For example, if we want to be accurate, it must be pointed out that your heart and lungs don’t EVER “do the work” of getting your bike up the hill, they just supply oxygen to the muscles, and the muscles ALWAYS do the work, whether they do it at a rate that can be sustained or a rate that can’t be sustained. If the muscles aren’t straining too hard, the hard and lungs can deliver oxygen as fast as it is used, and the rate of work can be sustained. In terms of accurately stating what happens, simply gearing-down does not, by itself, create aerobic exercise that substantially counts as exercise. The rate at which work is being done is what matters, and simply gearing-down does not gaurantee that substantial work is being done. For example, a car can run in first gear on flat ground at redline and still only put out 1/200th of the available horsepower. That’s a mechanical example of gearing-down just for the sake of gearing down, and you can do the exactly the same thing on a bike if you do what was literally written in the previous post. Like I said, I’m sure the intended meaning was in regard to a constant, difficult-to-maintain level of work output, but that’s NOT what was actually stated in words.
Point Taken
The wording might have been clearer on this end, had I slowed my spin down long enough to consider the degree of efficiency (or efficacy). 
Hmmm. I’m thinking how simply strapping a bungeed tennis ball to the hull increases the resistance to the point where it becomes more about power than aerobic fitness. Technique: I’m also thinking about how in road racing, you can spin your way to the top of a hill starting in a lower gear and increasing cadence, or how you can judge the distance and race your way to the top, banging down a gear at a time to stay at a constant cadence while losing as little momentum as possible. The efficiency of my narrower hulls enables me to stay at a higher HR and maintain it over a given distance for a given power output, similarly my road racing bikes are more efficient to this end versus my mountain bikes.
Interesting to get into paddle blade discussion as well, noting that marathon event paddlers tend toward smaller blade sizes (less surface area) to maintain a higher cadence over a longer period of time, versus your sprint blades that require huge amounts of energy output to achieve greater speed (provided you can 'spin’the blade up to speed).