Follow these steps:
1) Stand diagnoly (northwest) from the SAILOR who says "The ship set sail."
2) Press right and start at the same time.
3) Do NOT close your menu but save the game and turn it off (then back on).
4) Get on one of your pokemon that knows SURF (which you will need in your party before trying this cheat).
5) You'll be on top of the SAILOR! Press down to go behind him and down once more and then you'll be back by the S.S. Anne! If you want to, you can look around the ship or battle any remaning Trainers that you didn't beat before.
Romney '08!