I would support Cheechako's suggestion - hydraulic tank for steering/autopilot.
The very stiff hose will be reinforced for pressure and the valve on the top is likely to be a bleed valve or similar. A royal pain in the whatsis if you need to do anything to the system.
On the off chance it is just a pressure accumulator for your water
system, the valve in the top is a schraeder valve and again, needs access to pressure test/top-up if needed.
On the other hand, it could be one of those widgets that do their secret thing to something, somewhere - sorry!