Abstract: A marine observatory craft for viewing of underwater locations includes a plurality of planing type hulls with viewing ports associated with each hull. There is also provided a plurality of transverse bulkheads dividing each hull into a plurality of separate internal compartments which are sealed from each other. In each internal compartment there is provided one or more ballast tanks which function as seat supports wherein each internal compartment includes a viewing area having one or more of the seat supports. A control cabin superstructure is located above the plurality of internal compartments. There is also provided a device for selectively flooding the ballast tanks whereby the craft may move in a traveling mode wherein the viewing ports are located above the waterline and an observing mode in which the viewing ports are located below the water line. The flooding device includes a pump, a plurality of valves for controlling the flow of water through flow conduits, an inlet and an outlet.