The answer is found in Exodus chapter 14. From verse 10 the instant cause of their fear is shown: "And when Pharaoh drew nigh, the children of Israel lifted up their eyes, and behold, the Egyptians marched after them; and they were sore afraid."
They had good cause to be afraid, so they cried out to Moses with what we might call today, "a knee-jerk reaction". But note from verse 12 that those speaking up were the same ones who had been reluctant to leave Egypt in the first place. They admitted to having told Moses, while they were still in Egypt, to leave them alone to continue serving the Egyptians.
It wasn't until Moses - full of faith - cried out to God about the apparently imminent problem (i.e. threatened massacre) that God told Moses to lift up his rod and to stretch out his hand over the sea, and then it would divide so that the Israelites could go on dry land through the midst of the parted waters (vs. 16).
Then the pillar of cloud removed to the back of the camp of the Israelites. That would be the point at which fear would be replaced with wonder and awe. The Egyptians were stopped in their tracks. Then, as Moses obeyed God's commands, the waters parted and the nation began to go through to the other side.
However, it would still take faith to go between the towering walls of water on either side. They had to have faith that God would keep that mighty east wind blowing all night long till the vast company had made the crossing. Once they began to step out in faith, there could be no turning back!
Hence Hebrews 11:29 stating that it was by faith that they crossed the Red Sea. It was only faith in God that resulted in their natural (understandable) fear being replaced with confidence that God truly was acting miraculously on their behalf, and so they stepped out in faith. Prior to that, they could step nowhere as they were utterly hemmed in! But when they saw God open up a way of safety - miraculously - then their faith enabled them to make the crossing.
That is how their fear and complaining can be balanced with the faith that followed.