I once heard a rumor about their being a Mew hidden behind the SS Anne with no way to get to it, but I personally don't think it's true. I wouldn't know for sure though, because I usually don't cheat on games unless I've beaten it and have nothing better to do. (Ever since I migrated all my Ruby Pokemon to Diamond, I've been experimenting with how far I can get using Walk Through Walls without ever getting my first Pokemmon. I made it all the way to Mossdeep once!)
But I'm still using my Red Version, so I've never tried any Mew glitches yet, but when I'm done with it, I'll give it a shot. I doubt it will work, but hey? What've I got to lose? I've always wanted to try riding the boat after the Pokemon League, anyway.