Successful Fishing

Dave and Tom are fishing on a lake. After about an hour when neither of them had a bite, Dave decided to row to the middle part of the lake; where they were very successful TOM: This is a great spot for fishing Dave we'll have to mark this spot so we can come back here tomorrow.
DAVE: I know, we'll put an X on the side of the boat to mark the spot.
TOM: Don't be silly Dave that's no good we might have a different boat tomorrow.