Documentation
From the US Coast Guard Documentation Center's FAQ section at the link below. "The official number assigned to documented vessels, preceded of the abbreviation "NO." must be marked in block-type Arabic numerals at least three inches high on some clearly visible interior structural part of the hull. The number must be permanently affixed so that alteration, removal, or replacement would be obvious and cause some scarring or damage to the surrounding hull area. "The name and hailing port of a recreational vessel must be marked together on some clearly visible exterior part of the hull. The vessel name of a commercial vessel must also be marked on the port and starboard bow and the vessel name and the hailing port must also be marked on the stern. All markings may be made by any means and materials that result in durable markings and must be at least four inches in height, made in clearly legible letters of the Latin alphabet or Arabic or Roman numerals. The "hailing port" must include both a place and a State, Territory, or possession of in the United States. The state may be abbreviated." What you consider "clearly visible" is up to you but I don't think in the bilge qualifies. The requirement for permanence means removal would cause "scarring other damage." There is a wealth of information at the NVDC web site which is worth a long look.
