Shmot Exodus 2:5 Pharaoh's daughter went down to bathe, to the Nile, and her maidens were walking along the Nile, and she saw the basket in the midst of the marsh, and she sent her maidservant, and she took it.
Rashi
her maidservant: Heb. אֲמָתָהּ, her maidservant. Our Sages (Sotah 12b), however, interpreted it as an expression meaning a hand. [The joint from the elbow to the tip of the middle finger is known as אַמָּה, hence the cubit measure bearing the name, אַמָּה, which is the length of the arm from the elbow to the tip of the middle finger.] Following [the rules of] Hebrew grammar, however, it should have been vowelized אַמָּתָהּ, with a dagesh in the mem. They, however, interpreted אֶת אֲמָתָהּ to mean her hand, [that she stretched out her hand,] and her arm grew many cubits (אַמוֹת) [so that she could reach the basket]. [From Sotah 12b, Exod. Rabbah 1:23]
In summary although there is room to interpret this event as a just a natural event, the sages leaned in the direction that indeed a miracle took place to save the infant, Moshe/Moses.
Batya took note of the miracle and made the right conclusion.