There are debates about if it was the gulf of Suez or another body of water.
Some say that this was a shallow body of water, denying this was a miracle. This couldn't have been the case since:
there were walls of water when they crossed. Exod-14 > v29
Pharaoh and his men were drowned in the waters afterward: Exod-14 > v28
God had a great plan to save Israel, that no man could have achieved
God made it look like the Israelites were bewildered, lost in the wilderness. Exod-14 > v3
God sent the pillar of cloud, and divided the sea.
No man could have done these things.
When we are in a suspenseful, dramatic situation we often respond in Fear.
This is how the Israelites responded here. Exod-14 > v11
They were not walking by faith.
It is dangerous to view the world by our own present circumstances, forgetting God's promises.
Notice that there is something quite ungodly about what they are saying.
Ultimately they are saying that God failed.
They blamed Moses, but ultimately their grievance was against God Himself, not just His messenger Moses.
This is the same way that we often respond, today, with unbelief
Moses told them to fear not:
Usually it's not very helpful to simply tell someone don't be afraid. They'll respond something like I can't help it. Instead we need to give someone a reason not to be afraid. And this is what Moses does next.