Adding weight to a kayak is the last thing I'd ever do
First thing I'd do is add pool noodles (four on each side) inside the hull at the rear. Adding pool noodles will not improve the dry buoyancy of the kayak, but will improve the kayak's buoyancy if it takes on some water in the hull.
To make your seat more comfortable
--get some Lexel from Lowe's and Lexel a closed foam kneeling pad and Lexel it directly to the bottom of the seat (not to the kayak)
I used clear Lexel to adhere a $6 kneeling pad to the bottom of my aftermarket kayak seat(adhered underneath the seat, not to the kayak) I put the handle hole where the tailbone goes. Gluteal area and low back much more comfortable after hours of fishing. I also considered a high end gel cushion, but they were much pricier and heavier . Before adhering the cushion on the underneath side of the seat; I put extra Lexel all around the edge of the cushion to make it water tight.
If you've had too much BBQ recently and are a bit heavy, dropping 10-20lbs would make a big difference to how the rear of the kayak sits in the water.