There are two ways to do this without taking it out of the water. The most obvious is to dive under the boat. If the board isn't fully retracted into the keel, you might be able to grab it and shake it to see if you can free it. It could be stuck because the pendent shackle is caught, or a few barnacles are blocking it. If the board is swollen with water, then there is really nothing you can do about this with the boat in the water. You will have to pull the boat out of the water and let things drain and dry out (you may need to drill drainage holes in the edge of the board to let this happen, but that's another thread).
If you don't want to dive or if the board is fully retracted, you do have another option. Go to the hardware store and buy a three foot long, 1/4 inch steel rod. It might cost a couple of dollars. The pendent line comes into the cockpit via a hole in the bridge deck. Remove the plastic grommet that surrounds this hole. You will see that the opening for the pendent line is quite a lot bigger than you might have thought. Push the steel rod into this hole, past the pendent line, until you feel the rod hit the edge of the stuck centerboard. You can now try and free the board "from above". I recommend tapping the end of the rod with the least amount of pressure that is successful. You don't want to really whack at the rod; you are striking the relatively vulnerable edge of your centerboard, and you don't want to crack it. I have used this method about a half dozen times over the past ten years to free my centerboard (Oday 23). It has never failed, nor have I ever damaged my centerboard.
A couple of caveats: Be careful pushing the rod through the conduit. I have heard tell that the conduit in some boats is not a hard pipe (as it is in my Oday 23), but a flexible hose at some point in its length. You don't want to puncture that hose below the water line.
In order to spread the load and lessen the chance of damage to the centerboard and to the conduit, I make a "pad" for the end of the rod out of duct tape. You could use other things as well, but you are limited by the diameter of the conduit.
Full disclosure: I have never damaged the board, but last summer I might have broken the shackle that connects the board with the pendent. The board stuck, and I used the rod. I freed the board, but quickly realized that the pendent didn't control the board anymore. I dove under the boat and found the clevis pin from the shackle to be gone. I don't know if the pin failed and was the cause of the jam, or if I broke the pin when I went to free the board. Since I never impacted the pin or shackle before while freeing the board, I am inclined to think the pin/shackle failed and caused the jam itself.
Good luck.