One thing to keep in mind is that a ship's name is a proper noun, which is why it's technically correct to leave off the definite article. Think of it as being like a city. You don't go to "the Los Angeles", you go to just "Los Angeles", and with people you don't talk about "the George Washington", it's "George Washington". The same applies to a ship using the same name. What happens in everyday use is that people go with whatever sounds best. It's a matter of speech patterns. Some names just sound better in English with "the" in front, some don't.