Both require living in some sort of pressurized container, unless you assume we could live in underground caves(but those can't be very deep so probably not good enough). Obviously the underwater one would require more engineering, but probably still less than one needed for Mars.

That depends on your perspective. We have the technology to go to Mars right now; we do not have the technology to go to the bottom of the Marianas Trench. It is a much more hostile environment than Mars is.

However, if you assumed that we were able to go the bottom of the ocean, transporting supplies there would be much easier than transporting supplies to Mars.