Well, remembering my own experience sitting on a high canoe seat with a high-angle stroke and a 230-cm double-blade paddle, I still can’t imagine a person with a long enough torso to require 240 cm, or even 230, when sitting low as is the case in a kayak. Picture where your lower hand is relative to the surface of the water during your stroke. There’s no need for the blade to be deep below the surface, and I have not doubt that it IS deep below the surface unless you are keeping your lower hand unusually high (which would be incorrect).
Second, the average sales person at the average outdoors store isn’t likely to know much, regardless of how much experience he says he has. Be skeptical of these guys. I could give several examples of so-called “experts” in local outdoor shops here who don’t even know enough to realize that they know next to nothing at all, in spite of the fact that some are quite experienced in their own little worlds.
Here’s one thing to keep in mind. Beginning paddlers, or at least this is true for males, tend to feel good about paddling really hard, and they tend to perceive hard effort as giving the effect they want. I would suggest not falling into that trap on two counts. First, pushing your boat at a speed that is faster than it efficiently moves takes exponentially more energy than going just 1 or 1.5 mph slower, and this is a VERY difficult thing to perceive at first without the aid of a GPS to reveal your true speed. So avoid that kind of paddling even if it feels like you are going faster, because your extra speed is far less than you probably think. Second, from a leverage standpoint, a longer paddle is working against you. For a given amount of force applied by the blade against the water, you will have to apply a greater force to the shaft with a longer paddle than with a shorter paddle. Since force applied by the blade to the water translates to boat speed, regardless of how that force against the water is produced, it makes no sense to use a paddle that requires you to pry harder on the paddle to develop the same force, and a longer paddle makes it necessary to do exactly that. Again, don’t be fooled by the paddle power as perceived through your hands and arms and upper body. That’s not a correct perception of the force being applied to the water. I can promise you that a shorter paddle that doesn’t seem to “dig in” is actually producing the same force against the water, and probably more, than is the case with your long paddle. You just haven’t had enough practice yet to know it…
When canoeing, I always carry a shorter, sturdier paddle just for use in shallow water. Sometimes when I’ve been paddling hard all day and I’m just badly fatigued, I’ll switch to that shorter paddle just to reduce my own output effort while maintaining the same cruising speed. Sure, my stroke is a little less picture-perfect with a paddle that’s technically too short for me, but it’s definitely easier on muscles and joints.
If you still aren’t picturing this, here’s an example to explain it. If you wanted to lift a shovelful of dirt with the least amount of muscle effort, would you want the shovel to be short with the blade being quite close to your “lifting” hand, or long so that the blade is three feet away? Oh, and think of this while considering that your hands are the same distance apart from each other in both cases, just as should be true with different-length paddles. If you still can’t picture this, get two shovels and try it for real. The blade of the shovel holding a glob of dirt against the force of gravity is analogous to the blade of your paddle pushing against the water, and the less distance there is between the paddle blade OR shovel blade and the hand that is closest to that blade, the less effort is required on your part.