I don't know what a pokéwalker is (though I can guess) but if there are screws that let you take the housing apart, you can try opening it up so that everything inside can dry completely and then put it back together and see if it works. This has worked for me on several electronic devices like PDAs and pedometers and such that have gotten dropped in water or otherwise very wet.
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"In the end, we decide if we're remembered for what happened to us or for what we did with it."
-- Randy K. Milholland
Avatar=WWI propaganda poster promoting victory gardens.