I want to build a drift buoy to send off to see if it could make it across the pacific, using ocean currents at particular depths as a conveyor and pressure sensor to determine depth. I will have a 'report' routine where it will surface and get a GPS location, charge the batteries with a solar cell, and transmit its data points and location to me using ULF. Obviously I want the thing to be using little-no power when it isn't collecting data or transmitting...
Now that is what I call an ambitious project. The part that would scare me as not likely is the 'call home' using ULF (ultra low frequencies). I came across some ULF systems in the military and they took pretty high power and very long antenna. Certainly it's possible as that's how US nuke subs got their launch orders even while submerged, but it's a pretty expensive power consuming method to 'call home'. I think most applications like yours use satellite uploads to send their data, but that also has expensive and complexity.
Good luck, and keep us posted on progress.
Lefty