Answering my own question based on recent experience.
As Aravona says in their comment many have drain holes in the seat area. I bought a sit on top, kayak with drain holes in the seat.
I took it to the lake and, put it in the water at the dock. I weigh about 180 pounds, and the kayak is rated at 350 pounds. I carefully got myself in the kayak and kept my feet completely dry. But when my butt hit the seat, water shot up through the drain holes and soaked my back side. Also When I paddled harder, it seemed like more water came in. I didn't really dwell on it, but I think the bottom of the seat and the drain holes in the seat are at or near water level when I am in the kayak.
I don't know if it was my entry style (sat down to hard?) or if it is a design issue with this particular style kayak.
I was wearing dry on the fly, long pants. Even gettting out of the kayak at the dock, I kept my feet dry until I stood up and the water from my soaked butt, ran down and soaked my water shoes.