An underwater launcher of a vehicle in a launching tube outside the pressure hull of a submerged launch platform is unaffected by launch depth. A muzzle cap at one end of the tube communicates with ambient water and a ram plate at the other end communicates through openings with ambient water. Rigid elongate segments extend inside the tube between the cap and the plate, and a launch mechanism connected to the platform and tube has an expansion chamber sealed from ambient water and contains a gas driven turbine rotating a pump-inducer communicating through the openings with the ambient water. The pump inducer is adjacent to the plate to simultaneously displace the plate, elongate segments, vehicle, and cap in the tube and eject the vehicle. The plate, elongate segments, and cap decouple from the vehicle as it leaves the tube and safely sink away from the launch platform.