Nephi reminds his listeners of an incident from sacred history in which Yahweh gave power to Moses to open the waters of the Red Sea. Therefore, Yahweh could also give Nephi power to prophesy. He did not choose this image haphazardly. When Moses, a true (and unquestionably accepted) prophet, opened the Red Sea, it resulted in destroying the army of a very powerful but wicked nation and preserving Yahweh’s righteous people.
Obliquely, Nephi is using that metaphor to predict the destruction of the Gadiantons. They have proclaimed themselves and their cities powerful, but so did the Egyptians of Moses’s time. They have declared their army strong (implied in v. 8), but so was the Egyptian army. The Egyptians were wicked because they did not follow Yahweh, and the Gadiantons are similarly wicked because they do not follow Yahweh. Moses was a prophet to whom Yahweh gave power. Nephi, with that same power, has declared that the same destruction will come upon the wicked Gadiantons as came upon the wicked Egyptians.