The story of that salvation went before them. Indeed, this is a powerful, graphic type of the salvation which He worked for all men through Jesus Christ. Both 'salvations' required, and require, trusting in God. Same trusting, same faith required. As if this is not enough evidence, consider why the Lord did not take the Israelites to Canaan by the most direct route.
Salvation is an ongoing process of spiritual refinement.
You see, physical miracles aren't enough. Such don't develop real, abiding faith that is independent of the physical senses. The Israelites needed not only to observe the mighty works of the power of God but also to depend upon Him in a practical way for the very necessities of life. Ultimately they needed to understand that all would be accomplished not by their own ability but by God's own power. This was clearly a statement by YHWH that the same power which defeated the Egyptian army and brought them safely out would provide the water necessary for life.