As G-d promised, the sea split and the Jews passed through it to safety. The Egyptians, including Pharaoh, followed them into the dry seabed, but G-d let the water return to its natural state, drowning them all. The Jewish People's first stop in the desert after the Sea of Reeds was Marah.
They came to Marah, but they could not drink the water from Marah, for it was bitter; that is why it was named "Marah" ("bitter"). The people complained to Moses...
The words "for it was bitter" literally mean, "for they were bitter." This may be understood to mean that the people, rather than the water, were bitter. In this context, the whole phrase reads, "they could not drink the water from Marah for they were bitter."
When we are bitter, everything tastes bitter. Only when we ourselves are "sweetened" with positive attitude fostered by Divine consciousness can the water--as well as everything else in life--begin to taste sweet as well.
—from Daily Wisdom 3