When Moses put his arm out at God's command, the waters of the Red Sea parted and were frozen in place on either side of some dry land. Moses by faith with over 3 million people crossed the sea in approx 1445BC.
The Red Sea had divided and congealed. When they had crossed safely over, the water came back drowning the chasing Egyptian armies.
This was a continuation of miracles that God did as He brought His people the Israelites back to the land that He had promised them many centuries ago.
The Egyptians will know that I am the LORD and also the people feared the LORD and put their trust in him and in Moses his servant yet they still grumbled against Moses and God many times over the next 40 years.
By faith the People passed through the Red Sea as on dry land; but when the Egyptians tried to do so, they were drowned.
The Red Sea divided and congealed
21 Then Moses stretched out his hand over the sea, and all that night the LORD drove the sea back with a strong east wind and turned it into dry land. The waters were divided, 22 and the Israelites went through the sea on dry ground, with a wall of water on their right and on their left.
23 The Egyptians pursued them, and all Pharaoh's horses and chariots and horsemen followed them into the sea. 24 During the last watch of the night the LORD looked down from the pillar of fire and cloud at the Egyptian army and threw it into confusion. 25 He made the wheels of their chariots come off so that they had difficulty driving. And the Egyptians said, "Let's get away from the Israelites! The LORD is fighting for them against Egypt."
26 Then the LORD said to Moses, "Stretch out your hand over the sea so that the waters may flow back over the Egyptians and their chariots and horsemen." 27 Moses stretched out his hand over the sea, and at daybreak the sea went back to its place. The Egyptians were fleeing toward it, and the LORD swept them into the sea. 28 The water flowed back and covered the chariots and horsemen–the entire army of Pharaoh that had followed the Israelites into the sea. Not one of them survived.
29 But the Israelites went through the sea
on dry ground, with a wall of water on their right and on their left. 30 That day the LORD saved Israel from the hands of
the Egyptians, and Israel saw the Egyptians lying dead on the shore. 31 And when the Israelites saw the great power the
LORD displayed against the Egyptians, the people feared the LORD and put
their trust in him and in Moses his servant.
and also see Exodus 15:8
Where Israel crossed the Red Sea under Moses is an interesting point one commentator suggests it was across a 13km path in the Gulf of Akaba the right arm of the Red Sea so they could enter Midian (Arabia). Mt Sinai is where Moses saw the burning bush is also in Arabia. Egypt at this time extended control all the way to the Gulf of Akaba with military strong holds.