Sunken Anchor

Weapon (war pick), uncommon (requires attunement)
  Whilst attuned to this weapon you have advantage on checks and saving throws made to avoid being knocked prone or moved against your will. You have a +1 bonus to attack and damage rolls made with this magic weapon.
  Anchor. As an action you can point this weapon at a point on a surface within 120 feet, causing an ethereal chain to run from you to the surface for the next 10 minutes, or until you dismiss it as a bonus action. For the duration, you can willingly move closer to the point, but a weight of over 10,000 pounds or a DC 30 Strength check is required to move you further from the surface if you are not willing. On a successful Strength check, or if more weight is applied, the chain snaps and the effect ends. The effect ends early if you ever willingly move more than 120 feet from the surface, or if you lose consciousness.
  Hydrodynamic. Attacking while underwater doesn’t impose disadvantage on attack rolls you make with this weapon.
  Rare variant: Increase the bonus to attack and damage rolls made with this weapon to + 2. You can invoke the weapon to cast the riptide spell (save DC 15) once per day, recharging at dawn.
  Very Rare variant: Increase the bonus to attack and damage rolls made with this weapon to + 3. You can can invoke the weapon to cast the riptide spell (save DC 16) at will.