Abstract: A water craft which comprises a motor propelled floating portion which has two spaced-apart hulls which are connected by an elevated bridge. The water craft further comprises a submersible cabin which is arranged to be raised and lowered with respect to the floating portion of the craft, the cabin providing accommodation for underwater observers. The cabin is connected to each hull by fore and aft pivotable arms and is movable between a first position above the waterline of the floating portion of the craft to a second, submerged, position. The cabin is accessed from the floating portion of the craft when the cabin is in the first position and, in order to provide for fail-safe operation, the cabin is ballasted for positive buoyancy when it is submerged. The cabin incorporates a tower which is dimensioned to project above the waterline at all times, including when the cabin is sumberged to the maximum extent permitted by the pivotable arms.