Albert Barnes Commentary Exodus 7:15

"Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river`s brink to meet him; and the rod which was turned to a serpent shalt thou take in thy hand." — Exodus 7:15 (ASV)

He goes out to the water - The Nile was worshipped under various names and symbols; at Memphis especially, as Hapi, i.e. Apis, the sacred bull, or living representation of Osiris, of whom the river was regarded as the embodiment or manifestation. If, as is probable, the king went to offer his devotions, the miracle would have special force and appropriateness. It was also the season of the yearly overflowing, about the middle of June; and the daily rise of the water was accurately recorded, under the personal superintendence of the king. In early inscriptions the Nilometer is the symbol of stability and providential care.