I don't know a whole lot about it but, with Chesapeakes/retrievers they call this "Water Freaking". It usually gets worse and worse. My buddy from Cle Elum has spent a lot of time just letting his dogs do their own thing in the water and it has propogated with his male. He will get in the water and swim in one spot, dive, bark, thrash around and won't get out until Mike starts to leave.
I took him out training a few weeks ago and the dog made one water mark. On the next one, a live flopping pigeon, he swam towards it, turned away and went out in the pond and started his "freaking" thing. The part that is bizarre is that on the land marks,(the dog hasn't had a single live bird/training in two years) I stretched him out throwing walking singles to over 200 yards and he was pinning the marks. Drive, desire and marking skills are definately not the problem.
I don't know the answer to correction of the problem. It is possible that e-collar may help but, if it really is water freaking, I'm not certain about it.
I haven't run across too many trainers who have worked on the problem or been successful with a dog who is like my buddies dog.
This comes back to force fetching your dog properly IMHO. People have many misconceptions of FF but, I think this is a particular situation that a proper early FF eliminates.
Have you been through a Force Fetch program with your Boykin? FF and CC(collar conditioning)? They pretty much go hand in hand. How old is your Boykin?