Abstract: There is provided a floating caisson device for placing below and transversely to the bottom of a vessel hull in engagement with the lower portions of the hull side walls for providing a dry working space, especially for joining floating hull sections by welding, comprising a bottom channel with side wall members at the ends thereof, at least one side wall member being able to be brought into sealing engagement with the hull side wall, packing for sealing between the device and the hull, and ballast tanks for controlling the buoyancy of the device. Between at least one side wall member and the adjacent end of the bottom channel there is provided a flexible transition member conforming to the curvature of the hull for sealing engagement therewith when the side wall member is moved against the hull side, the packing being continuously extending along each side edge of the bottom channel, the transition member and the side wall member.