I've only docked a sailboat one time without an engine--I got lucky and didn't hit anyone, and didn't kill myself. So I guess I really don't need an engine, but it would be difficult to give it up.
I tried using a sextant in the lower reaches of Chesapeake Bay, and from my viewpoint, it was worthless. Of course, this may have something to do with my ineptitude, but I won't say this was the only reason.
The one item I rely heavily upon, whether on a powerboat or sailboat, is my GPS/plotter. The accuracy has been uncanny, and if it were not for unlit pound nets, day markers and crab pot markers, I would sail on the GPS day or night without any qualms. Because of my recent incident of nearly nailing a pound net in the middle of the night, one that was set illegally I might add, I decided to install broadband radar on the boat as well--just to be on the safe side.
So, if there were only one item I could keep, it would definitely be the GPS Plotter. I'm not at all good sailing by the seat of my pants, slinging a weighted line to determine the depth, and I sure as Hell don't want to rely on a sextant while bouncing around in 10-foot swells in the middle of the ocean.