About the trail starting from the boat pier : The Hafnarskarð pass, the way to go to Hornstandir first campsite, can be challenging if the fog surround you. The closer you go to the top, the harder it guet to know where is the trail. Be carreful to bring a GPS with full battery and/or compass and map, good orientation skills are a must.
Short story about that:
On a rainy and windy day hiking up the pass, I have seen plenty peoples going off the track without realizing it, I had to show them with a map and compass they where going the wrong way. They where following a phone with offline maps, it was leading them offgrid, straight to a cliff in order to cut back to the right trail... Don't follow only the GPS, please bring something to know your way that don't require battery ;-) !