Fisher F-PULSE Owner's Manual page 11

Operation after submersion in water
The F-Pulse has a waterproof membrane that lets sound out.
It is located inside a cavity under the vent next to the battery
cap.
After it is immersed in water, the cavity fills with water. When
on the surface, water will drain out of the vent. You may want
to shake the water out.
The pinpointer may exhibit a muffled sound while the
waterproof membrane is wet. Shake out the water and let
membrane dry; sound will return to normal.
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