One of my kayaks developed a leak in the back hatch when it was about 6 months old. It was in no danger of sinking but on return from a camping trip, everything in the back was well damp.
Over the last 10 years or so I have had to deal with this problem quite a few times. 2 times out of 3 it has been the back hatch. It can be difficult to track down the source of a leak. Kayak manufacturers do a pressure test. They use a hatch cover with a car tyre valve in it then pump some air in and soak the outside of the kayak with soapy water and look for the bubbles.
I have found the leak it is most often where the skeg cable enters the top of the skeg box in the rear hatch followed by where the holes are drilled for the end toggles in the front or rear hatches.
You can test the end holes by upending the kayak with the leaky hatch down. Either lean the kayak against a high wall or get someone strong to help )and it is best not to do this in a wind!) Fill the rear compartment with a decent amount of water from a hose and look very carefully for leaks. In my case all was dry so...
This time I wanted a quick repair. I could have used GRP but instead, I smeared Evo-stik Serious Stuff Ultra adhesive round where the cable enters the skeg box. This product is waterproof and even sticks to wet surfaces. It is much cheaper than Sikaflex 291 sold by chandlers and is just as effective. (£5.98 for 290ml out of Tesco compared with £12.95 out of Duncan).