Exodus 15

They have just witnessed an amazing thing.  God has saved them miraculously.  They all sing a song to the Lord, singing of what he has done and how great God is.  Miriam starts singing afterwards along with the women saying “Sing to the Lord, for he has triumphed gloriously;
the horse and his rider he has thrown into the sea”.

They're on their way and cannot find any water.  They make it to a place where the water is too bitter to drink.  Then, they start complaining, wondering what they are to drink since they cannot drink the water there.  Well, after Moses cries to the Lord, the Lord shows him a log.  Moses throws the log into the water and the water became sweet so that they could drink it.  This is immediately followed by a statute made for them by the Lord saying "If you will diligently listen to the voice of the Lord your God, and do that which is right in his eyes, and give ear to his commandments and keep all his statutes, I will put none of the diseases on you that I put on the Egyptians, for I am the Lord, your healer".  They need to follow God and his way, not their own.  As we will continue to see, they keep complaining and screwing up...