Mainly relates to engine AFAIK. I have mine done, a single Mercruiser, by Mercury agaent. Flush cooling system and top up antifreeze as necessary. Grease all greasers, inc prop spline. Inhibit engine internals and spray exterior with inhibitor. Cost about £100, then full service in spring. I had it done to ensure warranty all OK, but that not worth much now.
flushing things, new oil. No I don't think it is worth in the S of UK, prob a bit in N if on the freezing cold side or inland- but if not then must be up to date with maint...like oil and flushing things. If you use the boat, obviously not unused for ages so no "winterising" needed imho