Tap one side of the sight in the direction of the muzzle, then the other side, then back to the first side, and repeat until it slides off. It should work. I might try heat from a blow dryer if that doesn't work, but personally I never liked the idea of using water. If you do use water, make sure you dry the barrel off really well, including the inside of the bore then re-lube it with a little dab of gun oil so there's no chance of rust. Barricade is a very good rust inhibitor on guns.
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.