Moses has to lift his staff up and stretch his hand over the sea and divide it. Then the Lord drives the sea back by a strong east wind and makes the sea dry land. The people go into the midst of the sea on dry ground.
See also Hebrews 11 verse 29.
Thereafter the Egyptians too, with their chariots, go into the midst of the sea on dry ground. The position of the pillar of cloud has possibly been changed. But when Moses, by God’s command, stretches his hand over the sea, the waters return to their normal course and cover the chariots and horsemen.
Moses and the Israelites express gratitude for their redemption, in a song to the Lord.
See also Revelation 19 verse 1 to 6.
In the painting the accent is on gratitude for the redemption.
Isaiah 51 verse 10 and 11:
Was it not You who dried up the sea, the waters of the great deep, who made the depths of the sea a way for the redeemed to pass over?
And the ransomed of the LORD shall return and come to Zion with singing;
everlasting joy shall be upon their heads; they shall obtain gladness and joy, and sorrow and sighing shall flee away.
The deliverance from Egypt is a foreshadowing of the redemption of the believers by Jesus Christ. The water is, in this context, an image of judgement and death. The crossing of the sea is - like the Ark of Noah - an image of the Christian baptism.
Romans 6 verse 3 and 4:
Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? We were buried therefore with Him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.
See also Colossians 2 verse 12.
The old slave-life was left behind in the waters of death.
God intended His people to live as a holy, that is a set apart nation, after their deliverance from Egypt.
Later on God would give them His law as guidance for a life of gratitude and obedience. Alas, despite God’s faithfulness the people often fell into sin.
In the New Testament this behaviour of the Israelites is presented as a warning and example:
1 Corinthians 10 verse 1 to 6:
For I do not want you to be unaware, brothers, that our fathers were all under the cloud, and all passed through the sea, and all were baptized into Moses in the cloud and in the sea, and all ate the same spiritual food, and all drank the same spiritual drink. For they drank from the spiritual Rock that followed them, and the Rock was Christ. Nevertheless, with most of them God was not pleased, for they were overthrown in the wilderness. Now these things took place as examples for us, that we might not desire evil as they did.
The deliverance in itself was no guarantee for a holy life. Holiness requires a heart that is completely consecrated to God, and total surrender to the guidance of the Lord.
After 10 plagues, which the Lord brought on Egypt, Pharaoh finally let the people go. Moments later he regrets it and decides to go after them. The Israelites are in a plight; the sea cuts off their way ahead, the Egyptian army is chasing them. God's saving presence appears when the Angel of the Lord places Himself between them in a pillar of cloud, so that the Egyptians cannot approach the people of Israel.