Water will not necessarily destroy gunpowder or primers, but oil will. The recommended disposal method for primers is to soak them for several days in oil. The problem with your .22 ammo is that it is probably sealed well enough that soaking them in oilo won't gurantee that the oil will get inside. If'n I wuz you, I'd just take it to the range and shoot it. You might also contact your fire department and ask their advice in safely and legally disposing of old ammo.

Properly stored ammo lasts way longer than you might think. I have lots of WWII surplus .303 ammunition, and it shoots just fine. The accuracy may not be what it once was, but in my WWII surplus No. 4 MkII Enfield with open sights and my aging eyes it is hard to tell.

BTW, one way to get rid of old centerfire ammo that you aren't sure of is to use a kinetic bullet puller, knock out the old bullet, dump the powder and burn it. You can then dispose of the primer by the simple expedient of putting it into your weapon, pointing it in a safe direction and pulling the trigger. If the primer is still active, it goes bang and you're done. If it doesn't after a snap or two, soak the primed shell in oil for several days and throw it away with a clear conscience.