Therefore let us go up; let us be like unto Moses; for he truly spake unto the waters of the and they divided hither and thither (1 Nephi 4:1)Nowhere in the Old Testament do we read that Moses spoke to the water. But there is an ancient Jewish oral tradition where God tells Moses to speak to the sea.
Take the rod that I gave unto thee, and go to the sea upon Mine errand, and speak thus: 'I am the messenger sent by the Creator of the world! Uncover thy paths, O sea, for My children, that they may go through the midst of thee on dry ground. (Source)These oral traditions can often seem strange to us. In this case, for instance, the water responds by speaking back to Moses. But Jews cared more for the message than the historicity or any scientific explanation. After Moses has spoken, he also uses his rod and then the water parts.
Since Nephi explicitly stated that Moses spoke to the Red Sea and it is not found in the Old Testament, he was likely referring to this oral tradition. This may have been added to or changed over many years, but the element of speaking to the waters is there. Alternatively, the current Old Testament contains only an abbreviated version of the one found on the brass plates and the oral tradition originates from this extended version which was later partially lost.