I would think yacht-borne heli maintenance would be very similar to offshore heli maintenance.
Offshore operators have learned the hard way how to work in salt-air environments. Follow the manufacturer's manuals, of course, but this was the routine for one well-known civil operator:
Start with a bare-metal ship, and wash thoroughly with mil-spec soap. Prime thoroughly. Apply a thin bead of "pro-seal" to every seam and joint...wherever moisture can collect and create dangerous corrosion. Apply a solid, multi-coat paint job.
Apply pressure tape where frequent rubbing or abrasion will cause the paint to be breached (near fuel fill cap, cargo compartments, steps, etc.)
Wash airframe daily. Water wash engine daily. Soap wash engine one a month. Cover stacks and inlets.
Wipe the blades at least once a week with 50/50 mixture of jet fuel and WD-40.
And keep an eye on scratches and dings. They will morph into bubbly white corrosion in no time.
What seems difficult with yacht ops might be the lack of copious fresh water and the "pristine" nature of the helideck. I'd want to wash the ship down daily with good, fresh water. Is that practical on a yacht? On offshore platforms we just drenched the ship, water running all over the helideck into the pad sump drains.
The folks drinking the mint juleps below decks on some of these yachts might not think too kindly of that...
Oh, and I've never heard of spraying anything into the exhaust stack of a C20...would be interested to hear what that was about.