Sea Hag
From WikiRaider
The Sea Hag is an enemy in Tomb Raider V in the level Old Mill. It appears underwater, protecting a silver coin that sits at the bottom. If Lara gets too close, a cut scene will be triggered, showing Lara struggling to get away from the creature. This cut scene will keep on playing until Lara will get the item. Lara must wait until the Sea Hag gets further away, pick up the coin and swim away before it comes back.