I'd say that's a life ring holder next to the hook & I think the hook is for clipping in one of those floating lights that attach to a life ring
I carry a horseshoe aft with a light floating line (ex-climbing rope), one end is tied to the horse shoe & the other end is made off to the aft cleat, the bulk of the line is coiled ready for throwing with the horseshoe in case of MoB. That hook would hold the coil nicely.
The idea is that in the event of a MoB, slow down & turn back towards the casualty, stop engines & throw the rope & horseshoe. You can then bring the casualty to the boat. If you can't get the horseshoe to the casualty you can gently motor round so that the trailed line & horseshoe are within reach, or even just recover it all & throw again.
The point is to avoid prop damage to the casualty if you need to bring them to a stern boarding platform. Of course you do have to watch out for fouling the prop(s) in the floating line, but at least you can see it. In my own case I might be trying to recover the MoB under sail & short-handed so a simple way of sailing round the casualty with a trailing line makes more sense than it might for a MoBo, but it's an idea that may be helpful.