The cache is located at the Wetland Trail. It connects Puerto Villamil and the nearby land tortoise breeding-station. As all trails in the Galapagos National park it's open from 6am to 6pm, this time not protected by gates, but by the sundown. After that, it get's really dark out there.
You do not really need to leave the trail, except for a step on a rather huge patch of volcanic tuff under witch the cache is hidden. Other than that, do not stray from the path itself!
Thanks to Mettematros for providing a new spoiler photo: Spoiler