Yes, it was tried tested and fitted to aircraft of the Royal Naval Fleet Air Arm. One of the most famous of these was the Blackburn Buccaneer.
Below is an overview of the system fitted from a technical manual of the era.
MARTIN BAKER PATENT FULLY AUTOMATIC UNDERWATER EJECTION SYSTEM. JUNE 1965
The Martin-Baker patent underwater escape mechanism was designed to provide fully automatic escape facilities for aircrew who had to ditch their aircraft and were rendered unconscious on striking the water, It was capable of ejecting the airman from the aircraft, separating him from his seat and supporting him on the surface by his fully inflated life-jacket, no action whatever being necessary on his part.
The system consisted of a main air bottle open to water pressure via a stainless steel pipe to a convenient point on the aircraft skin. When the sinking aircraft reached a depth of approximately 13 feet the pressure on the valve diaphragm removed the restraint on the firing pin, allowing it to descend under spring pressure and fire the cartridge in the head of the bottle.
The compressed air is thus released and is piped to two places, the lower end of the ejection gun and the drogue gun trip rod release unit.
The air entering the bottom of the ejection gun forces the inner and intermediate piston tubes upwards, unlocking the seat and ejecting if from the aircraft but without firing the ejection gun cartridges.
The air to the drogue gun trip rod release unit forced a wedge shaped plunger out of its housing and in so doing releases the drogue gun trip rod preventing the gun from being fired. At the same time the plunger struck a linkage which withdrew the guillotine sear thus severing the link line between the drogues and parachute. The movement of the plunger uncovered a port in the unit allowing the air to proceed to the specially modified life-jacket inflation bottle via the personal equipment connector. The bottle was thus actuated and the life-jacket inflated.
As the seat rose on ejection the time-release unit was tripped in the normal manner and ran for a period of 1.5 seconds. At the end of this period the plunger descended and struck the harness release lever. On rotating, the harness release lever released the harness locks and at the same time it operated the C.0.2 bottle mounted on the starboard side of the seat, the gas being piped to two bladders, one behind the parachute pack and the other, underneath the dinghy pack.
The action of these two bladders was to push the occupant from the seat and, as the harness locks had been released by the time release unit and the parachute withdrawal line severed by the guillotine, a clean separation was initiated. The occupant was thus free to float to the surface unencumbered by an opened parachute and supported by his life-jacket until a rescue could be effected.
The system could be rendered inoperative by a handle attached to the main air bottle which, through a linkage, operated a slide valve. When the slide valve was moved to the inoperative position it blanked off the delivery ports to the base of the ejection gun and the drogue gun trip rod release unit. At the same time it opened a port so that, should the main air bottle inadvertently operate the air will exhaust to the atmosphere,
A secondary escape operating mechanism was fitted. On operating the emergency handle situated on the starboard thigh guard, the sear is withdrawn from the firing body which allowed the striker to go forward and fire the cartridge. The gas pressure impinges on the valve diaphragm which removes the restraint on the main air bottle firing pin thus initiating the escape sequence.
The underwater escape equipment in no way affected the procedure for an airborne ejection which could be accomplished in the normal manner. Standard items of the ejection seat which had been slightly modified for underwater escape included the ejection gun and the personal equipment connector. The former had a plug fitted to the lower end of the inner piston tube and held in position by two shear rivets. This had the two fold purpose of minimizing the amount of air required and preventing the compressed air from rupturing the diaphragm of the primary cartridge and possibly causing it to fire. A sear of a different design was fitted to the firing unit in the ejection gun breech, this allowed the firing cable to disconnect when the ejection gun extended, thus preventing the firing of the primary cartridge. The personal equipment connector was modified to include the air line to the life-jacket inflation bottle and incorporates a ball valve in the aircraft portion, held open when the seat and aircraft portions are connected and automatically closed to conserve the air when the seat and aircraft portions separate.
Pre flight Action, Airborne ejection sequence and Action after Flight are perfectly standard. Manual separation and free bale out in the event of damage to the seat was again similar to the standard guillotine seat.
One extra, safety pin was provided in addition to the standard safety pins; for rendering the emergency firing handle inoperative, or for placing in the sear of the secondary operating firing unit when necessary.
A spring-loaded protection flap, which covers the ejection gun primary firing unit, was attached to the upper rear face of the pilots ejection gun. Integral with the flap was a window for visual inspection of the correct attachment of the firing cables to the sear.
Incorporated in the main air bottle operating head was a safety slide which, when in the engaged position, obstructed the downward movement of the bottle firing pin. When the bottle was installed in the aircraft the safety slide was locked out of engagement by fitting a pin into the hole provided in the safety slide.