This is the creature schtick of the same name, Feng Shui, p 124.

You can easily move and breathe underwater. If you are an aquatic creature that is handicapped on land you need not purchase this schtick; Amphibian allows you to operate both on land and in water. In some cases, the schtick can be bought in forms relevant to other strange environments (such as deep space or the dense atmosphere of Venus).

If you lack this schtick, your Move is halved and all actions count as continuous actions in water. You can stay without air a number of sequences equal to your Constitution.