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.
Heading back to Lake Ontario for this summer. Now in Bermuda and heading to Long Island Sound in the first week in May. Then on to the Hudson and the Erie Canal.