A buoyant device has a body and an elongate tail. The body carries a payload. The tail is moveable, e.g. by pivoting, between a closed and an open position. This movement changes the position of the center of mass of the device relative to the center of buoyancy. As a result the device can move through the water, with the tail in the closed position, with minimal drag. However, when the tail moves to the open position, the body pivots in the water so that the body, and hence the payload is supported in a stable manner.