Undertow

You target a creature in the water with a powerful riptide dragging them below the surface unless they can succeed at a strength saving throw. Each round you can attempt to drag them under again, forcing them to repeat the save as long as you maintain concentration. Creatures dragged down by the undertow  can move on their turn but, whether swimming or moving along along the seafloor Strength (Athletics) checks they make to swim upwards against the undertow are made at disadvantage.

This penalty does not apply to creatures using freedom of movement.