I was going to give an updated answer to this question but I think Rocco's answer is still pretty sound advice. However, it is still possible to add to and expand on a few of the points he made.
If you want a dedicated motorcycle GPS rather than some improvised solution then you will have to be prepared to pay for it: Motorcycle navigators are significantly more expensive than standard devices used in cars.
The 2 most popular manufacturers are Garmin and TomTom. Garmin produce the Zumo series of motorcycle navigators, of which there are currently 5 models in production while TomTom have just the one model, the TomTom Rider.
As regards waterproofing, all the above models are rugged, purpose built motorcycle navigators and so are designed for all weather use.
Almost all modern GPS devices come with an internal battery so that is not so much of an issue any more. However, there is considerable variation in battery life among them, and, as regards the above mentioned motorcycle navigators, is between 3 and 8 hours.
Besides hiking, you should also consider whether or not you want to use the GPS in your car. The TomTom Rider, for instance, unlike Garmin's Zumo series, does not come with an automotive mount and power cable, and has no internal speakers. Garmin devices, on the other hand, can be transferred more or less seamlessly between car and motorcycle.