When this happens, the fin had as you said deep fisures between the blade and the head. I remember I was not far to the beach ( almost 1 km) and I manage to return using my harness I just put the harness from the rear strap of the leward rail and drift to the beach.
From this time I always check and double check fin, tendon, boom, and all the ropes, every time I go into the water.
Buy a new fin, the same Drake is a good fin, but as is a very powerfull fin, it's prone to be damaged when you fly it.
Now I have the same fin again from 2 seasons, without problems, and always have a backup ( in my case a Select R03Wide DC Medium) Not the fast fin in the market but as some one said " never go out without it".