Amphibious Movement

I notice that some units can't be made amphibious, they just walk on top of water.

Is amphibia a part of model or is there a way to make normal units amphibious?

Models for amphibious units need a special set of swimming animations for when the unit is in deep water; in that case, the origin of the unit is on the water's surface, while in shallow water the origin is at ground level.

I thought so, but I felt like asking just in case.