Removing Stuck Surfboard Fins

To free up stuck surfboard fins, pour some sub-boiling hot water over the base of the fin box to loosen up the accumulated gunk, sand and salt that has built up in there.
Then, take part of a towel, wrap it around the fin, wiggle it to loosen it, and finally pull back and down as hard as you  can to pull it out.
If necessary, you can also try firmly swinging a rubber mallet in the same direction you need to pull to help it along. Only use the rubber mallet if you aren't worried about damaging the fin and are unable to pull it out with your hands.
If that doesn’t work, then try a spray of WD-40 directly at all four sides of the fin box.
If that doesn't work, either find a surf shop that will help you out with their fin puller or buy your own.
A puller was the only way we could get stuck surf fins out of our 1970s Rick board, even though it had not been in the water much.
If you need to purchase a puller, we've had good luck with a brand called Finpuller (finpuller.com), which we find works well except on the really small fins.