A person says: "My plan is to set off at high tide"
and minutes later: "We'll ride the ebbing tide back to the ship."
Is it contradictory because, as I understand, high tide is rising of the sea level and ebbing tide is falling of the sea level?
They are setting off at high tide, when the tide is at its maximum and has ceased rising. It will then be ebbing [retreating from the land], so they will be riding the ebbing tide out to the ship.