A securing device comprises an axially reciprocable drive shaft which has a head adapted to pass forwards through a detent plate. The head is adapted to engage the plate after rotation. A cam slot is defined in a drive housing and is engageable by a cam follower carried on the drive shaft. The cam slot has two end portions extending parallel to the axis of the drive shaft and an oblique intermediate part such that the drive shaft is rotated when the follower moves along the intermediate part and the drive shaft moves only axially when the follower moves along either of the end portions. A subsea assembly including the securing device includes a fixed stab plate including a central collar, and a free stab plate having an aperture through which said collar extends, the collar including an chamber defined by the detent plate.