@John Tubb When my CB would get stuck up all the yard needed was a short pry bar to get it down with a quick flip of the bar at the rear of the CB trunk. Of course that was while hanging on the lift but underwater should work also. I tried my self underwater with channel locks and could not get a grip. I didn't have a boat ladder at the time so I never tried it again myself.
Here is what I would do. I would first take down some metal pallet strapping, the 1/2" or so wide stuff or something similar, about 18" in length. Wearing gloves I would try sliding it between the board and trunk on both sides. If it is clear, then I would know it is a pendant/shackle issue. If not either foreign material, swelling or crack.
Then I would try the small pry bar. Once down, if the strapping found a tight spot I would give that area good look and feel if I got the board down. Might tell you something. If it didn't find a tight spot concentrate on the way the line is tied to the CB. Shackle? Does the clevis pin feel longer than needed? Is there an eye splice or is it tied on with a knot? Or is it rigged another way? Is the pendant line oversized? (1/4" is fine, no more than 5/16" or it could be catching on the worn groove of the tube)
Remember, the trunk narrows near the top so anything that is none OEM may be causing you problems.
Good Luck. If you can't get it down, you can still sail. Just not as close to the wind without a bit of slippage.
Enjoy the 4th.