There are a few threads on it here.... but most car ones aren't waterproof.. and the motorcycle ones (unless you get used or close out or a hell of a sale) are often double the cost of the car ones...
Alternately you can get a non vehicle only one ... they are often a bit more rugged... the GPSMAP series is what I got... mine is waterproof, in fact it floats, it can be mounted on a motorcycle which you can get from Ram Mounts..... and best of all mine is one I can take camping or even up in a plane...
The GARMIN GPSMAP 60 and 76 series are really quite good... depending on the options you want (like altimeter or sailing maps etc...) you choose the model...
I got the Garmin GPSMAP 76CSx and love it.... it cost me about $270 a few years ago.... sure I can't play MP3s on it nor does it talk to me, but it does everything else very well... and the sensor system in it is very very very good compared to many other units...
I believe the GPSMAP 60 series were originally designed for the US Military too... and are in a bit more ruberized, rugged housing than the 76 series which is a combo unit which was designed with boating in mind since the case was designed to float...
There is not much difference between the two but the 76 has marine mapping options and floads... otherwise go for the gpsmap 60CSx it is cheaper by a fair amount.
or.... spend a bit more and get a larger dedicated bike one...
Garmin GPS are still among the best though....
Here is a link someone did on the GPSMAP 60 C (basic model CS and CSx have a lot more features and sensors)
http://www.kenrockwell.com/tech/gpsmap60c.htm