Huckleberry I've got an old one that has zero flotation, it was made in San Angelo and I promise I've sunk it one time. Bought it forty something years ago. It's called Lampro Company. Total fiberglass and only seven foot long. Looks more like a Canoe but is sold as a kayak. They had one that had a box in the center and the trolling motor clamped to the box that was open at both ends.
Also my son has a kayak that was sunk in the red mud that I recovered as only the bow was sticking out at lake champion. I found it with my Skeeter bass boat fishing so finally pulled it up and towed it in. Son put new hatches on it and his wife now fishes in it.
My Widerness Systems kayak is probably unsinkable though.
But the old ones are not unsinkable.