Frankly, the amount of LENGTH of the boat in the water changes less than you think.
I think the LENGTH of wetted hull is likely to change so little as to have that effect on speed be virtually indetectable. Slightly more significant (as mentioned in other posts) might be total wetted area. But again, even that difference in speed is likely to be hard to detect for most paddlers. I mean, those calculations which others refer to are (I believe) calculations for 'theoretical MAXIMUM (or optimum) hull speed'. Very seldom are you (as a recreational paddler, not an Olympics-capable racer) likely to approach pushing the boat toward that speed.