I am guessing something like what fishing trawlers use, stability down riggers but on smaller scale, something along the lines of a winged keel with open top type buckets/pods which act as sea anchors as the board tips.
As the board rolls up the pods act as a sea anchor, open top allows water to "flow" out on the down roll. The pods wouldn't have to be big so pretty stream lined/low drag for paddling .
well thats my guess....

but I do like the idea on a mini collins class sub filled with wallabies prop fwds for ballast....Go the Irish