An over-the-center toggle mechanism for actuating locking dogs for connecting and disconnecting two underwater members. The first member includes a locking notch and the second member slideably carries locking dogs for transverse movement for engagement and disengagement with the locking notch. A toggle linkage includes a first pivot connection connected to the dogs and a second pivot connection longitudinally actuated by a piston whereby longitudinal movement of the second pivot connections transversely moves the locking dogs. A resilient load spring acts against the second pivot connections and towards said first pivot connections for locking the second pivot connections in a past dead center position relative to the first pivot connections. The second member includes a tubular body having an annular cavity with a longitudinally reciprocal carriage mounted in the cavity and actuated by the piston. The second pivot connections and the spring are transversely movable on the carriage and carried by the carriage. Preferably the cavity includes seals for sealingly enclosing the cavity and includes a flexible member exposed on one side to the interior of the cavity and on the second side to the exterior of the cavity for compensating for a difference in fluid pressure between the cavity and the outside of the body.