Eel Skin

This spell only functions if the target is completely submerged in water.

You touch a willing creature. Until the spell ends, the target’s skin is grey, smooth, and glistening. The target’s AC cannot be less than 16, regardless of what kind of armor it is wearing, and the target gains advantage on any saving throw to escape or resist being grappled or restrained.

Credit- Dungeon Masters Guild: Underwater Campaign; Daniel Chivers