Why underwater hull paint is red

Why is the underwater portion of so many ship hulls painted red?

Because inexpensive copper-oxide antifouling paint is red and helps prevent marine growth.

The familiar red band below a ship's waterline is not primarily decorative. Traditional antifouling paint used copper oxide to discourage organisms from attaching to the hull. That compound naturally contributed the red color. Cost reinforced the convention because the resulting red formulation was an economical way to protect a large underwater surface.