I talked with the seller about this and he said this was common with this model and that it could be easily remedied by placing the kayak upside down, in sun, with a stick underneath it to push the depressed area out. Hmmmm.... Sounded reasonable. It always does when it's somebody else telling you how to fix something.
I've since learned this is a common problem with Tempests. The bottom side of the hull is the thinnest part of the kayak. Apparently, it is the least stressed part of the kayak and the Wilderness people decided they could lighten the overall weight of the kayak by reducing the amount of plastic they deposit here. The sides of the hull are of normal thickness because they take the bulk of the stresses encountered in waves and surf. I too have noticed this kayak flexes more than does other kayaks of similar length and was quite surprised the first time I loaded it by myself, how light it is - 59 lbs. It's much easier to lift than my 14.5 Carolina. That explained it. Still, the bottom does tend to oil can... I've not seen one yet that wasn't canned. It's just a matter of degree.
I've paddled the kayak a lot this season. The canned pockets don't seem to alter the speed much. I can paddle at 4 knots for extended periods and have no difficulty keeping up with my fellow paddlers. Yet, I know those irregular surfaces must create some additional drag and I frequently wonder if the kayak would not paddle easier if they were removed. But how? Some say pour hot water into the kayak and remove it quickly; others say sit the boat in the sun with the hull up and brace it underneath the canned areas so that they pop out; others say use a heat gun... None of these solutions are very practical.
Today, I decided to remove the cans from my Tempest.
WARNING! If you choose to use this technique, be extremely careful to not leave the lamp unattended as you could melt a hole in the bottom of your kayak.
The picture below is a before shot. In this boat, the canning is cellular, i.e., pockets appear at one foot intervals along the bottom of the boat with the keel line separating pockets on the two sides. All in all, there were four rows of depressions in my boat. Two rows were beneath the portion of the cockpit where I normally position my legs (imagine trying to rig a support so that the boat's weight rested on these buggers), one was in the hull below the day hatch (conveniently off-center), and one was below my seat. I didn't attempt to repair the one below the seat, because I don't have a clue as to how to remove the seat without damaging the boat.
If you decide to try this on your kayak, a few words of caution are in order. Do not leave the lamp on while you are not there to monitor the surface temperature of the plastic. You could severely damage your boat. Test the temperature of the plastic cautiously. The plastic could burn you. Use an oven mitten if you must work the plastic into position manually. Do not allow the surface of the heat lamp to touch the plastic hull. It could melt through and then you would have an even worse problem on your hands - how to fix that gaping hole in the bottom of your boat.
Now, if I can only figure out how to get rid of all those scratch marks on the hull...