Steel that is exposed to water and salt rusts quickly. If you were a shipbuilder, how would you protect a new ship? Explain?

I would electroplate the steel of the ship with a material that will sacrificially rust such as zinc or perhaps some titanium alloy, preserving the basal steel of the ship. I'm not an engineer so I don't know what material would be ideal. One would have to think of the costs to manufacture the electroplating surface, its abundance, its resistance to flow, the resistance it has to corrosion, and how damaging its corroded ions dissolved in the ocean would have to promoting algae blooms.

Pieces of solid metallic Zinc are attached to the hull of the ship. Zinc is more reactive than Iron / steel and corrodes preferentially, protecting the Iron /steel. The zinc piece (which is known as sacrificial anode) becomes porous and thins down in due course as metallic zinc transforms to zinc ions. These need to be removed and new zinc pieces attached to the hull from time to time. The iron / steel acts as cathode in the sea water electrolyte. Magnesium and aluminium have also been used to protect iron / steel. This is required in addition to normal painting.

use zinc parts on the hull under water to produce what is known as a Anode to dissipate built-up charges reducing oxygen molecules influence . Good paint, of course

Make my ships out of wood, coated with pitch. No rust with wood.

Keep it inland in a hangar, away from water.

Yeah. You should probably go with a combination of "coating" and "painting."

By covering it with primer