These Israelites were scared and murmuring, and on the surface it would seem that there were reasonable grounds for fear. After all, they were not trained soldiers and they were being hotly pursued by a fierce army. But they had witnessed the power of God in Egypt and this same God was fighting for them. They couldn't have fought Pharaoh's army alone even if they tried, so all they could do was be still and wait for God's deliverance.
Exodus 14:30 Thus the LORD saved Israel that day from the hand of the Egyptians, and Israel saw the Egyptians dead on the seashore.
I can think of so many worldly fears that on the surface seem really reasonable to have. It's hard not to be fearful of an economic crisis, diseases, natural disasters, or even of violent crimes. As Christians, we're not promised immunity from these things. What we are promised is deliverance from eternal death if we're faithful (Revelation 2:10). We may have to endure this scary world first, but we don't have to endure it alone. We have access to an Almighty God through Christ Jesus. What a comfort to know that if we are His, we can take our fears to Him and He'll deliver us in the end.
John 16:33 "...In the world you will have tribulation. But take heart; I have overcome the world."