SRD:Pearl of the Sirenes
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Pearl of the Sirines: This normal-seeming pearl is beautiful and worth at least 1,000 gp on that basis alone. If it is clasped firmly in hand or held to the breast while the possessor attempts actions related to the pearl’s powers, she understands and is able to employ the item.
The pearl enables its possessor to breathe in water as if she were in clean, fresh air. Her swim speed is 60 feet, and she can cast spells and act underwater without hindrance.