A lot of water could go into the cockpit with the transom open. It could drain out quickly too, but would pull the stern down pretty quickly. If you had concerns about getting pooped it would be pretty easy to install large drains through the transom.
Can't see in the photo but how much depth is there between the cockpit sole and the cabin opening? This is a vital concern because it is more important to keep water out of the cabinthan out of the cockpit.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.