Complex sentence: Tide tables give the height of the tide above a chart datum making it feasible to calculate the depth of water at a given point "and" at a given time by adding the charted depth to the height of the tide.
Split into simpler sentences: Tide tables give the height of the tide above a chart datum. This makes it possible to calculate the depth of water at a given point "and" a given time by adding the charted depth to the height of the tide.