Why Red?

I may be wrong, but I believe it is simply left in the rust/red color of the anti corrossion primer. No sense painting the lower below waterline area with expensive paint when it cannot be seen below the water.

Lon

I'd gotten the impression that red was the color of the paint like substance used the deter barnicale etc. Also red is one of the most visible colors. If a ship was listing or capsized the red would easily be see by sources of aide, another ship or plane.

This is THE reason that hulls are painted red. I work at Norfolk Naval Shipyard where we have guys who specialize in this. I asked the experts why this color and this is what they told me.
Back when ship's hulls were first painted for anti-fouling (sea critter repellent) the best paint for this was a high lead content paint similiar to red lead primer. This paint has a naturally occurring red pigment and the lead killed seagrowth. Prior to this paint being laid on, a primer was applied to allow better adhesion to the hull. It was a purer red lead paint.
In later years, better anti fouling paints became available and different primers were used having a different pigment. The outer coat remained red because as it wore away it would expose the colors of the primer coats beneath it thereby giving notice that a new outer coat of paint was needed. It was kept a red color supposedly because it showed up better underwater when the checks for wear were made and because of Navy tradition. (The Navy is big on tradition)
The black stripe between the red bottom paint and freeboard (above the waterline) paint is called boot-topping. It is a paint formula that is made to resist sea water and air exposure. Submarines are painted in this paint on their topsides and are now being painted in this color where they used to get painted red. US subs are now black all the way around and from stem to stern. It's supposed to be cheaper than masking them off and painting the bottoms red.
Some ships that I've seen in dry dock are painted a light blue-green like a copper oxide kind of color. They are always experimenting with different anti-fouling paints.
Anyway, this is what our painters (Metallic corrosion control technicians, sorry) told me so there you have it.

Darn! I'm disillusioned! ...And all this time I thought it was just to: 1) tell the bottom from the top; 2) provide a fairer, easier target for submarines to hit; 3) let the crew know when it was sinking; 4) just be "more pretty"; or 5) feng shui concerns.

"Should we prosper it shall be as is our custom...by Miracle!"

Another answer to this question might be, "Why not?"

I seem to recall that the Italian Navy, in WW2, painted their ships green below the waterline. Any idea why? Was Italian anti-fouling copper-based?

Most of the colors are based because of the quanity of metals used in the paint. Zinc, copper, chromium, and lead will give off primary colors such ad blue, green, red, or yellow. That would be why you may see some bottoms green, blue, or red. Subfixer explains it best. the color depends on what is cheapest, available, and gets the job done. With the advent of polymers, colors may change since now they can make a material to resist sea water and organisms that has no metal in it, or may be a co-polymer such as what is used on subs and will be black or nuetral. You also may see a chocolate brown / dried blood color, this was because a clay would be added to the paint.

Red bottom paint has been around for quite a while - I believe since the late nineteenth century. Some of the earlier paints contained particles of copper, which had been demonstrated to be an effective material for discouraging marine growth. I used to work in a maritime museum, and had occasion to restore an old model whose bottom had been painted with genuine, full-size "copper bottom paint." As I remember (this was twenty years ago), we found a marine supply company that was still selling the stuff. It looked coppery in the can, and for a little while after it was brushed onto the model's wood hull. As it dried, though, the surface of the paint turned dark red. If you scratched it, the scratch would look copper; that color would change gradually to dark red. I imagine surface oxidation was responsible.

Green anti-fouling paint has been popular in various countries at various times; I believe a number of ocean liners and cruise ships in the 1950s and '60s had green bottoms. I have with my own eyes seen, in Life Magazine, a color photo of the U.S.S. Iowa on the launching ways. Colors in old magazines are, of course, not to be relied upon, but this one seemed to be pretty accurately rendered. The color of the Iowa's underwater hull in that shot was a distinct, though dull and somewhat brownish, yellow.

On a related subject - my father, who served on board an attack transport during WWII, was quite emphatic in his recollection that during the war the Navy issued a metal primer that was a pale, slightly greenish yellow (zinc chromate, I imagine). Chipping paint and repainting were, of course, constant activities on board every U.S. warship during the war - and a coat of primer was always supposed to precede the camouflage color. I once built Dad a model of his ship on which I'd put lots of little pale-greenish-yellow spots, to represent the areas where the working parties were chipping and repainting that day. Dad said I got it right. I've used the same technique on several other WWII American warships; the yellow spots always bring grins of recognition from WWII Navy vets.

The first rule in the Navy, "If its nots breathing, then it is being chipped, primed, or painted".

I was a young Marine pfc on the "gator freighter" Arneb back in 1968, the swabbies sent us down in the hold to paint the decks red lead. They gave us 15 gallons of paint and one paint brush. We were a working party of 7 0r 8 guys. What to do? We found three mops (swabs), dumped all 15 gallons on the deck and smeared it around. What a thick gooey mess, the navy bitched and moaned, wanted or officers to do something, but we had to go do marine things like play invade the beach. They never asked us to paint again. Memories of "red lead".............
G.L.