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.
Back in the water in Grenada - with new main and #2 and cockpit canvas (Santa came early). Will spend the winter and early spring in the Caribbean and then head to Bermuda and the northeast US. Still trying to decide if we will bring the boat to Canada, either in 2015 or 2016.