As mentioned above the 6D probably fits your requirements best due to the larger sensor, however I believe it is a plastic body so may not suit the ocean and desert environments you mention. the 5D mark II (
second hand) will probably be in a lower price range and has better weather sealing (as I understand it, I might be wrong) and a full frame sensor.
If you want to push the boat out the 5D mark III is the latest model that has the aluminium body/weather sealing and full frame sensor.
A much cheaper option could be the EOS-M. It has the same sensor as the 700D (a crop sensor) but in a much smaller/lighter package. It has a metal body, but probably not proper weather sealing. However it's currently selling for £199 at Argos, including kit lens and flash, and there is a nice 11-22mm lens available in the M mount for just over £300. It might be a nice little low budget alternative.
Anyway, to compare the ones you mentioned:
- 700D - cheapest of the 3 you've listed - Plastic build - no top display - has a flip out LCD screen - crop sensor
- 70D - bit better build (aluminium body I believe) - top display - has a flip out LCD screen - crop sensor
- 6D - Plastic build - top display - full frame sensor
Cameras by other manufacturers are available...
Edit: this might help you compare high level specs:
http://snapsort.com/compare/Canon-EOS-6D-vs-Canon-EOS-70D