Abstract: A hydrostatically releasable coupling assembly for releasably securing a buoyant object to a vessel with a lashing strap. The coupling assembly includes a pair of lashing plates, a cooperative latching mechanism carried by the lashing plates for releasably coupling the lashing plates together and a composite plunger assembly coupled to one of the lashing plates for movement between a release position and a locking position. The plunger assembly has a first diameter portion shaped for release of the latching mechanism for movement from an engaged position to a disengaged position when the plunger assembly is in the release position, and a second diameter portion formed of plastic and shaped to retain the latching mechanism in an engaged position when the plunger assembly is in the locking position.