I like monocles and used mine many times, usually having it in the pocket of my foul weather
coat or sailing jacket. I also used several small binoculars too, which I used only during the day.
I found the smaller (about the size of a short thick sausage or 1 inch X 4 inches) monocular 8 x 21 comparable to the small binoculars 8 x25 for LACK OF ease of use, but the key difference can be the Field of View (FOV). A narrow field of view, small exit pupil and low eye relief can make these smallest optics difficult to use when trying to locate a distant object or follow a small moving object while you are on a moving boat, and if you wear glasses.
The smallest size monoculars (e.g 8 X 21) are most comfortable for having in a pocket because of their minimal size and weight, but they are smaller than optimum for viewing though, and generally have a much narrower field of view and smaller objective lens, so not as bright. They would NOT be good to use during low light conditions or at night, as they don't transmit enough light.
At the other end, a typical "spotting scope" will be too large, unwieldy, and too high magnification, and probably not waterproof and without a built in compass.
I also used small 8 x 21 or 25 binoculars (Nikon and others) too. I used them, but would not recommend them, because there is something better that does not cost much more.
Since the OP is only using one eye, the monocular is ideal, being less expensive and lighter and smaller than a comparable binocular.
, this is the one I would and will get:
I like this one compared to many others after looking at all specs, features (waterproof, compass, etc.), BAK4 glass type, accessories (it comes with a hand strap and attached lens cover), good customer reviews
, lifetime warranty, and a competitive price
But, as with any optics, it is a good idea to put them in your hand and up to your eye(s) and then look through them at something distant and if you wear glasses, with your glasses on too. . You might not find this item small enough or expensive enough etc. but, based on the published specs and features and intended use on a boat, this is what I would get.
Good luck on your choice.