ebb and flow of the tide
Refers to the natural rise and fall of the sea level; a set phrase often used in descriptive or literary contexts.
The ebb and flow of the tide is caused by the moon's gravitational pull.
On this coast, the difference between high and low tide is large.
満潮 specifically means high tide, while 潮の干満 refers to the whole cycle of ebb and flow.
Compound of 潮 (tide) + の (possessive particle) + 干満 (ebb and flow). 干満 itself is a compound of 干 (ebb) and 満 (full, high tide).