First question is are you worried about it being water proof?
I have been running a Nuvi 855 and during those times it does rain, a plastic bag would work fine...for me i say let it rain let it rain.. i run unprotected routes and waypoints...its not like it is going to be under water 3 meters down with pressure, however if it is under at 3, the rain will be the least of your worries
Willie
A word of caution - I have the nuvi 765t, ran into a quick rainstorm, and thought the same thing. Kept up a decent pace so as to keep it safe from direct rain; but it still got wet. worked for the rest of the day, but would never turn on again. I am debating sending my "out of warranty" unit back to Garmin for a $99 repair. still cheaper than the Zumo, but a pain the ass never the less.