A massive blue pearl set in an elegant silver pendant, the pearl of the deep is an item of legend captured from far below the waves, stolen by intrepid adventurers from the hall of a storm giant at the bottom of an ocean trench. It glows with a pale blue light when the attunement process begins.
The wearer can choose a spell from the Water Domain spell list up to half the her character level (sum of class levels) to be contained in pearl. It can be cast as if the casters own spell slot. The spell slot is regained and can be reassigned at a long rest. The wearer may choose intelligence, wisdom, or charisma as her spellcasting ability.
A little something to flavor a character with water magic, even if not a casting class. Things like this give your players more agency with which to interact in your world.