Steel Wrack Cloak

Seaweed Monstrosity. You can use an action to transform into a Shambling Mound for up to 1 hour. The transformation otherwise functions as the Polymorph spell, but you can use a bonus action to revert the transformation. You can use this feature once per long rest.
  Immutable. While you are wearing this cloak, you have advantage on any saving throw against spells or effects that would change your form against your will.
  False Appearance. While you are underwater, you can use a bonus action to expand the cloak and envelop yourself with loose strands of seaweed. While you remain motionless, you are nearly indistinguishable from a regular seaweed plant.