The Amulet of ranging is an amulet which boosts the player's Ranged maximum hit. It is exclusively dropped by Aquanites which are found in the last section of the Fremennik Slayer Dungeon, just past the Kurasks, and can only be attacked with a Slayer level of 78. Aquanites are assigned as a task by Sumona, and Duradel/Lapalok. To receive aquanites as a task from Kuradal you must spend 50 slayer reward points to unlock them.
The examine text is a reference to a necklace made using a shark's tooth, which is a necklace that is commonly known as being worn by surfers, however it is not possible to make one from the Shark's tooth obtained from the Fishing Trawler.