An underwater breathing device, such as a snorkel, may include an exhalation valve. The exhalation valve is configured to produce positive end-expiratory pressure in the airway of a user of the underwater breathing device. The exhalation valve includes a plate defining an exhalation port and at least one chamber port, an exhalation conduit connected to the exhalation port, and a flexible membrane that is sealable against a surface of the plate. A lower portion of the exhalation conduit is divided by a septum which divides the exhalation conduit and the exhalation port into a first exhalation port connected to a first exhalation conduit and a second exhalation port connected to a second exhalation conduit. The flexible membrane is sized and positioned to be capable of sealing the first exhalation port and the second exhalation port.