Or HaChaim on Exodus 17:5/אור החיים על שמות י״ז:ה׳

1 עבור לפני העם, "pass before the people, etc." Perhaps G'd was aware that the thirst amongst the people was very great and that they would be in danger of their lives very shortly. This is why He told Moses to pass in front of them on a path which suggested to them that he went to find water in order to quench the fire of their thirst. He was not to take a route unknown to the people and not visible to them. He was also to take along the elders to confirm the people's impression that Moses was going in search of water and not to isolate himself in prayer.

2אשר הכית בו את היאור, "with which you have struck the river Nile." This in itself would be a remarkable miracle seeing that the very staff which had made the water of the Nile fetid, undrinkable, and turned it into blood, would now produce water for the Israelites to drink. This would also deflate the irreligious heretics who denied the fact that Moses' staff possessed miraculous powers.