Abstract: A floatation device is disclosed that comprises a container 10 containing a liquefied gas, a gas chamber (60, FIG. 2) and a remotely operable device 29. The remotely operable device is switchable between a closed state in which fluid communication between the container and the gas chamber is prevented, and an open state in which fluid communication between the container and the gas chamber is enabled and vaporization of the liquefied gas charges the gas chamber with gas. The liquefied gas may be liquid nitrogen and the container may be heat insulated with an insulating vacuum cavity. A buoyancy unit (40, FIG. 2) which comprises a rigid enclosure (42, FIG. 2) defining an interior volume and a flexible diaphragm (55, FIG. 2) that partitions the interior volume into first and second chambers is also disclosed.