It is often assumed that water resistant devices do not have humidity in them, though this is often not the case. Garmin, like many companies, creates devices with a level of humidity that is most conducive to manufacturing and overall device performance. As a result, all electronic devices, not just devices made by Garmin, can experience temporary condensation, moisture or fogging inside the device when you transition to an environment with a significantly different temperature. To decrease the chances of this occurring with your device, make the transition in temperature as gradual as possible.
If condensation occurs, move to a warmer environment to promote evaporation.
If the moisture does not go away, there are dried water spots on the inside of the glass or there are visible puddles of water (more than beads of condensation) visible in the device it is possible the seals that provide water resistance have failed.
Being mindful of the signs of water ingress and the signs of natural condensation will help you determine if the device may need to be repaired. For more information on water ingress visit Garmin's water rating website.
Garmin Water Rating Website - http://www.garmin.com/en-US/legal/waterrating