well the tide duznt go out and in at the same time each day usaly low tide is about 3mins after the day befor so low tide can be at 3:00 one day and 3.03 the next but thair are 2 low tides in 24 hours. so the captan of this ship could have looked at the tidel chart of the week befor by acadent then could have lefft the boat to come back and finde out that he has no water lefft under his boat. very easy thing to do if ur in a rush .