After the sea split and went back to normal, the Jews move on through the desert and they encounter a place with only bitter water. In 15:22-23 the pessukim tell us, "ויסע משה את ישראל...ויבאו מרתה ולא יכלו לשתת" - Moshe urged the Israelites to travel... and they came to Marah and they were not able to drink...
Rashi tells us that Moshe had to push them to travel from the edge of the sea. He explains that all the jewels and wealth of Egypt washed up on the bansk and the people were busy scavenging the wealth and they did not want to leave. Moshe had to tell them that enough was enough and it was time to move on.
I find the contrast stark. They finish the story of splitting the sea and immediately begin the story of the bitter waters.
I think, maybe, part of the reason hashem took them to Marah right now was specifically because of the greed they displayed at the edge of the sea. They preferred to sit there collecting money and Moshe had to force them to move.
Hashem brought them to Marah to show them that all that money they were so worried about does them no good anyway. All the money in the world could not buy them a drink of water. They needed to improve their emuna and be less concerned about the money.
Thinking about this some more, I developed it a bit further....
After the events at the splitting of the sea and then the bitter waters, the rest of the parsha deals with a number of other stories; the manna, the fowl (slav), the water coming from the rock, the battle with Amalek.
All these are events that are completely dependant on emunah. They were all there to teach Bnei Yisrael emunah in Hashem and that even when they put forth effort it is really Hashem making their efforts successful.
So, they showed their greed by preferring the money. Hashem showed them the money is not the most important. Then hashem taught them multiple lessons in emuna.