Ebb and neap, for two

The answer to the crossword question “Ebb and neap, for two” is TIDES. Tides are the rise and fall of sea levels caused by the gravitational pull of the moon and the sun. Ebb tides occur when the water level recedes, while neap tides are the weakest tides that occur when the sun and moon are at right angles to each other. Tides are important for navigation, fishing, and other marine activities.