When one is not an amphibian, how does one become amphibious? Innovative drive.
As humans, its far easier for us to adapt to life underwater than it is for a fish to be out of water struggling in a bipedal world.
Consider yourself lucky because your soil stompers can be used to navigate the ocean's currents and tides without the need for robot flippers. But now come to mention it, robot flippers might be pretty cool..