You are confronted with the poor design of an otherwise very nice piece of hardware on your boat. To set off frightening you first. The repair of my passarelle was about 14k $.
The good news for you is that your rotary actuator is not yet completely seized up as mine was. That means you can probably still take it apart. But to get to that thing is quite the job. First you need to get all the hydraulic hoses off and sensors off. Before you do that make sure the passarelle sticks out 90 degrees. Now you can remove the top part. It's heavy, really heavy. About 100+ kg or so. once that part is off you need to get off the base part. Again not so easy as removing the bolts still leave the chaulking to stick. Once the base is loose you can bring this to a hydraulics shop and the can start rebuilding it.
A new rotary actuator is around 6k$ including transport.
My passarelle was fully overhauled, all seals were replaced all cylinders overhauled, a brand new actuator and the whole thing was repainted.
I made a small change to prevent the ingress of (sea)water into the rotary actuator by putting a cover over it and putting a think layer of lanolin grease under the cover. The problem is that the way the actuator has been designed is that part that rotates sits inside a large cylinder. There is a narrow seam between the moving parts on the top side of the actuator. Opacmare made a poor design choice here to allow seawater to reach this seam and slowy but surely get into the mechanism. Once that happens it starts rusting (it is all mild steel) and at some point it is stuck to the point you will never be able to disassemble it.
If you need more details let me know.