When you are out on the trail, and your hike just turned into a muddy downpour mess of an afternoon, you'll want to make sure that you are carrying a GPS that is rated to handle the weather.
Most handheld GPS units are rated to the IPX7 waterproofing standard; a pretty good standard that keeps the unit sound in the face of whatever a typical hike bike, run or other outing can throw at it. Some handhelds are rated to the IPX6 standard; which is a lower standard that doesn't offer the immersion capability that the IPX7 does...... Immersion capability? Yes, see below:
IPX6 Waterproof Standard
Heavy splashing and rain - This test sends water at all angles through a 12.5mm nozzle at a rate of 100 liters/min at a pressure of 100kN/m2 for 3 minutes from a distance of 3 meters. Must not fail or show water seepage.
IPX-7 Waterproof Standard
Puddle, stream, beer cooler and splash rated - Protected against water immersion - Immersion for 30 minutes at a depth of 1 meter.
I have had my GPS units out in the driving rain for hours and have dropped them in muddy puddles without issue on the IPX7 standard. I like it and it works for almost anything you can throw at it on a typical day.... except dropping it overboard on a boat..... and luckily that hasn't happened.