The Garmin nuvi 500 and 550 claim to be waterproof. price starts at under $200
Here is another word of warning then.
Both the Garmin Nuvi 500 and 550 are 3.5" screens, about 25% smaller than the 4.3" screens.
Biggest concern though is the fact that both these units were introduced in 2008 and will come shipped with pre 2008 maps. If you buy online, you will still have to pay Garmin for map updates to get you to 2011 maps at the cost of up to $75 depending on the units model. I know on some units Garmin has a "map Guarantee" that you will get a one time map update for a brand new unit but I think that applies to units bought from Garmin or one of it's certified dealers. Just buying a 3 year old unit from Amazon, Ebay, ect may not qualify to get this and it may not even apply to these units.
And these units are marketed towards off road activities such as camping, hiking, ect. They include topographical map information on the maps.
What does this mean? It means that little 3.5" screen that should be displaying street information such as street names, exit #'s , ect, will instead display mostly topographical map information such as Terran, elevation, ect.
These units while being waterproof may be great for someone who spends a lot of time off road and still helps you get there on the road? They will be less helpful when driving on pavement due to the lack of information on the map.
You can't have it all. If they were to put all the information from a "Street GPS" and add all the topographical information, the screen would be so "Busy" you wouldn't be able to read it while riding.
The reason why they are still so expensive is because they are "Specialized" units just like the Zumo's. You pay a premium for the ruggedness of them even though the components inside are identical to it's more fragile sister units that are not specialized.