Why in 2014 is the ocean still such a mystery. We overcame obstacles to space travel 50+ years ago but can't figure out water.

We actually can do some pretty cool things in water. We get oil from miles below a surface that is miles below the waves, we explore at tremendous depths, and we lay cables that stretch the length of the oceans.

It's true that there's still a lot left to do, and we certainly could do a lot more. But the reason we seem to be behind compared to space has less to do with pressure than with light (or electromagnetic waves more generally).

The reason we know so much about space is that we can see really far, and what we see contains a lot of information in the form of light spectrums, positions, speeds, etc... Also, most of the things we look at are really big, and stand out clearly from the background.

Water, on the other hand, blocks all of that. It scatters light, scatters heat, and makes info gathering a much more personal and in your face endeavour.

Another reason people forget about is atmospheres of pressure.

When you go into space you need to design a ship that can take -1 atmosphere of pressure and remain structurally sound. (14.6 psi give or take) Or not at all if it's unmanned. Electrical conductivity isn't impeded by a vacuum(err... maybe I mean that it is and so the board won't short circuit... you know what I mean) so you can have a non-pressurized environment with functional computers.

Every 10 meters of water increase the psi by 14.6(ish). The deepest portions of the ocean can reach up to 11000 meters. That means that you have to deal with 1100 atmospheres of pressure to reach the deepest areas of the ocean. (close to 16000 psi) Even in an unmanned probe you need waterproofing for your electronics that works at those pressures.

So to recap. Space requires you to build for -14.6 psi. Deep ocean requires 16000.