Basically, the COD is the boat's title. It is the overriding ownership papers in the United States. It's not necessary to have a boat registered with the US Coast Guard, if you stay in US waters. However, anytime that you travel into international or foreign waters it is necessary. If you don't the COD and you travel outside US waters, it's the same as traveling to a foreign country without a passport or any other type of identification. Not a situation I want to ever be in.