diver lift on boat

I need a diver lift to take divers from a water level work platform to the main deck of a dive salvage vessel 42 inches above. It should be a simple system of aluminum rails (I beams) attached to the transom in the vertical position. There should be a pair of trolleys or carriages that ride the I beams up and down. The trolleys should further servce to support the deck of the diver lift. I would like to lift the platform hydraulically or electrically. I cannot tolerate high posts on the deck because high vertical posts would impact other operations such as crane swing. This means that the lifting mechanism must reside below the deck. There are many examples of boat lifts that would be suitable, particularly with regard to the trolley mechanisms and their use of Delran rollers. I can draw this out when someone responds.