The Ejection Site  Underwater Ejection  Probably the rarest form
of ejection, ejecting while submerged. As odd as it may sound,
it is feasable and has been done successfully. Once submerged,
it is virtually impossible to open an aircraft canopy against
the pressure differential between the water and the air in the
cockpit¹. Once the cockpit is full of water, it might be
possible to slowly push the canopy open and exit the craft, but
the amount of time necessary for the cockpit to fill would allow
the plane to sink below the depth a pilot could survive. The
pilot's oxygen mask is optimized for use in thin atmosphere
conditions, and cannot be counted on to provide breathable
oxygen under any depth of water.  The above factors mean that
there is only one significant option available to a pilot once
the craft becomes submerged- eject through the canopy. The same
forces that prevent a pilot from opening the canopy manually
would prevent the jettison charges from pushing the canopy
safely out of the way of the ejec