http://rucool.marine.rutgers.edu/atlantic/about_gliders.html

http://spectrum.ieee.org/robotics/industrial-robots/remotely-piloted-underwater-glider-crosses-the-atlantic

How does an underwater glider "fly" ?

Most underwater vehicles, like submarines, use a spinning propeller to move
around in the water. Propeller driven vehicles are fast, but they also require a
lot of energy to maintain their speed. Smaller vehicles like the glider only
carry enough battery power to drive a propeller for a few days at most.

Instead, underwater gliders move around by changing their buoyancy, that is they
change their density such that they alternate between more dense and less dense
than the surrounding ocean water. This change in buoyancy causes the glider to
rise and sink in the ocean. The glider changes its density by moving a small
piston forward and back that increases and decreases its volume. You may
remember that you can calculate the density of an object by taking its mass and
dividing that by the object's volume. Since the mass of the glider remains
constant, all we need to do is change its volume. A small change in volume
(about a half cup of water) is all the glider needs to change its density enough
to rise and sink in the ocean.

As the glider goes up and down, its wings give it a forward motion just like the
wings on an airplane glider, which is why these robots are also called gliders.
But airplane gliders can only "glide" as they fall downwards due to gravity.
Underwater gliders can glide forward both as they rise and fall.