He turned the sea into dry land - The Red Sea, when he brought his people out of Egypt, Exodus 14:21. This was an illustration of his power, and of his ability to defend and deliver his people. The terror in that case, or that which was “terrible,” was the overthrow of their enemies the destruction of the Egyptians in the Red Sea - thus showing that he had power to destroy all the enemies of his people.
They went through the flood on foot - literally, “through the river.” It is probable that the reference here is to the passage of the river Jordan, when the Israelites were about to pass into the promised land Joshua 3:14-17; thus combining the two great acts of divine interposition in favor of his people, and showing his power over streams and floods.
There did we rejoice in him - We, as a nation - our fathers - thus rejoiced in God. See Exodus 15.