If your stainless steel cleats, stanchions and other topside hardware are rusting and difficult to maintain, use Ajax, Bon Ami, Soft Scrub, Comet, or other such abrasive based cleaner as a polish. Rinse well and then apply a coat of auto paste wax.
Rust rings. The bottom of metal cans leave behind rust stains under the galley sink and other storage places especially in the salt water environment. Clean the stain with Soft Scrub. Prevent them from coming back by affixing masking or duct tape around the bottom rim of each can.
Stubborn stains on fiberglass can be removed with acetone or MEK (methyl ethyl ketone). Its harsh and evaporates quickly, but it won't harm the gel coat if you put it on then wipe it off quickly.
Attack rust stains on your stern drive by brushing on some Naval Jelly. It contains a mild acid, so hose it off quickly. To help keep rust away from your engine and stern drive in the future, regulary lift the engine box cover and let fresh air circulate.
Rust stains on sinks and drains will lift right out if you rub the surface with a cut lemon. For stubborn stains, you'll have to keep working at it until the stain fades.
Here's a terrific way to stop rust rings before they get started. Rust Ring Ridder