I assume your talking "boating" in a salt water environment? If so, there is nothing that I am aware of that you can do to carbon or alloy steel, that will prevent it from corroding/rusting in that type of environment. Even the majority of steels that are labeled "stainless" will rust/corrode in a salt water environment.
Generally if it's corrosion resistance you want, for a salt water environment, choosing a steel with the highest percentage of chromium is a requirement. Personally, if you don't feel comfortable working with that type of steel, then I'd refuse the request. OR, if you want to make it out of the type(s) of steel you feel comfortable working with, make is crystal clear to the family friend that IT WILL RUST, and there's not a whole lot they can do about it, except to take extreme care of the knife.