The Israelites went into the midst of the sea on dry land, and the water formed a wall for them on their right and on their left.
And the water formed a wall for them on their right and on their left: Allegorically, too, we need to be protected from the floodwaters of life on both the "left" and the "right." When the evil inclination attempts to lead us astray from the proper path, there are two tacks it can take.
It can attack from the "right": When we are doing well, financially and otherwise, it attempts to convince us that we have no need for Divine assistance. It says to us, "when the Jews were in the desert, without anything at all, they needed G-d's help for food and sustenance. But you are doing quite well all on your own. Your charm/business sense/trading savvy is all that you need. Why bother with old-fashioned rituals and constrictions?"
Or, it can assault us from the "left": When things aren't going so well, it attempts to convince us that we have no time for eliciting Divine assistance. "This is no time for restrictions," it says. "As soon as things get better, when you've got the time and the money, then you will be able to worry about such things as Shabbat observance, ethical business behavior, and the like. But right now you've got to work. You're in the rat-race, and you need to concentrate solely and fully on getting ahead."
We therefore need G-d's protection on both sides, for only with it can we be assured of successfully traversing the turbulent waters of life and staying on the tried and true path of the Torah.