Alan and Barry go fishing

Alan and Barry hired a boat and went fishing some distance out at sea. They struck it rich, catching fish after fish until the boat was filled to the brim with beautiful catch.
They headed off for home as happy as a pig in muck. Barry said "We will have to come back to that same spot again and do some more fishing next week.
Alan agreed but a thougtht dawned on him "But how will we know for sure we will find the same spot in this big ocean"? he asked. "Thats not a problem " replied Barry, with a cunning smile on his face. "I happened to have a felt tipped pen on me and I put a mark on the side of the boat"
Alan relaxed for a while and a thought struck him and anxiously asked. "but what if we don't get the same boat"?