Sharkrazor Mantle

While wearing this magic shark-skinned cloak, you have a swimming speed of 60 feet and can breathe underwater. When a creature successfully grapples or restrains you by touching you, that creature takes 1d6 piercing damage. A creature takes this damage again at the start of each of its turns while it maintains this grapple or restraint.

In addition, while wearing the cloak, you have advantage on Wisdom (Perception) and Wisdom (Survival) checks made to find or track a creature that’s missing any of its hit points, and when you attack a creature you can’t see that’s below its hit point maximum, you make the attack as if you can see it.