This illustration is from Sandy Petersen’s excellent Field Guide to Cthulhu Monsters. It was published by Chaosium and while I do not play the Call of Cthulhu game, I did pick up the book for the great illustrations. Shown below is a Shoggoth in water.
Being buoyant in water, the Shoggoth takes its natural aquatic form of a sphere. This shape must aid the Shoggoth to conserve its energy. Lovecraft said that Shoggoths were, “fifteen feet in diameter when a sphere. They had, however, a constantly shifting shape and volume -“. Thus, when active, the Shoggoth is an ungulating mass of living matter, not limited to a specific shape or size. However, in its resting state in water or as a means of moving on land, the Shoggoth is a sphere.