[Anthony]

Hm. Why is walk on liquid [15] while amphibious is [10]? Both will allow you to move across the surface of a body of water at full move, but amphibious also allows you to move underwater and eliminates penalties for operating underwater.

The big advantage of walk on liquid appears to be that your only point of contact is the bottom of your feet. This means you can protect yourself (and your possessions) from harmful effects of liquid by just wearing appropriate shoes, but amphibious really doesn't address how much of your body needs to be immersed.