Pour your brains here on how to build this contraption...

[Enthalpy]

An example of a seaglider, Jura from the Scottish Marine Robotics Facility, meant for maximum efficiency to operate for months:
sams.ac.uk and explanatory video Lhp368-NMFo

Elegant laminar shape at 0:51, resembling my sketch but in true life
Jan 26, 2013 in this thread

It uses a bladder to control the buoyancy. No details provided, so a motor powered by a battery supposedly transfers the liquid between the bladder and the pressure vessel. The Oceanic temperature gradient would save this energy, but the equipment needs some electricity anyway, and months of autonomy are already a lot.

Cute piece of engineering!

[Bufofrog]

I would think and easy way to do this would be to have a battery bank in the glider and solar cells to charge the battery.  When the batteries are charged to a set amount a motor will move a piston and spring arrangement that compress a gas or air and latches which will pull water into a cylinder making glider sink.  When it reaches a certain depth or the batteries are low the piston can be released so the gas expands and pushes the water out of the cylinder.  The glider will then return to the surface where the batteries can recharge.

[Endy0816]

2 hours ago, Enthalpy said:

An example of a seaglider, Jura from the Scottish Marine Robotics Facility, meant for maximum efficiency to operate for months:
sams.ac.uk and explanatory video Lhp368-NMFo

Elegant laminar shape at 0:51, resembling my sketch but in true life
Jan 26, 2013 in this thread

It uses a bladder to control the buoyancy. No details provided, so a motor powered by a battery supposedly transfers the liquid between the bladder and the pressure vessel. The Oceanic temperature gradient would save this energy, but the equipment needs some electricity anyway, and months of autonomy are already a lot.

Cute piece of engineering!

I was thinking you could do this though drag and power to change buoyancy might pose scaling issues.

See submarines doing this already to a degree when blowing or flooding the ballast tanks, depending on the angles the stern and fairwater planes are at.

 

I would think a compressor would be needed to get the change in buoyancy. Might be other options though.