Open your map and head to the small dark colored island near Anvil. On it is a small boulder near the edge facing out into the sea. Once you find it, jump straight out into the water and swim straight down. Note: Use a water breathing spell or enchanted item for this, as it is some distance down. There will be a skeleton with the Fin Gleam helmet just above it. It is an enchanted glass helmet which gives night eye, water breathing, and detects life for 20 feet.