Here is the full list of all 17 Isles of Sea and Sky achievements.
Visit the Sanctum.
Visit the Tidal Reef.
Visit the Raging Volcano.
Visit the Frozen Spire.
Visit the Stoney Cliffs.
Befriend the sea turtle!
Awaken the Stone Elementals.
Awaken the Water Elementals.
Awaken the Fire Elementals.
Awaken the Air Elementals.
Collect every Rune Stone.
Collect 11 of each elemental gem.
Sleep for a total (cumulative) of 1 hour.
Visit 15 different islands.
Collect 25 stars.
Collect 50 stars.
Collect 100 stars.