After reading the portion of Az Yashir, we continue to read one more piece - the arrival of the Jews in a place called Mara after they complained they had had no water for a few days, before the aliya is broken down.
The whole piece is about the splitting of the sea, so why do we not just stop the reading at the conclusion of that portion? Why do we continue and read about the events at Mara and not put them in the next aliya?
The splitting of the sea was the height of inspiration. I can try to picture the water rising up into a wall. I can try to picture the water crashing down on the Egyptians behind the fleeing Jews. I can try to imagine the awesomeness of it. I think of Niagra Falls and find that amazing. Think of a sunset or any natural phenomenon that you find inspiring, and the splitting of the sea was that a million times multiplied. It even says a maid servant was more inspired at the splitting of the sea and had a higher level of prophecy that did the greatest of the later prophets, Yehezkal ben Buzi.
But inspiration is not enough. They were so greatly inspired, but right away they started kvetching again as soon as things got uncomfortable.
What happened to the great emunah they had attained and displayed by the splitting of the sea that the passuk even describes ויאמינו בה' ובמשה עבדו? What happened to the great inspiration? They were without water and suddenly forgot the power Hashem has to provide? They forgot all they saw at the splitting of the sea and in the desert leading up to it and in Egypt?
They wasted the experience of the Kriyas Yam Suf. They were inspired, but they let it slip away. Inspiration is nice, but it must be actualized into some sort of concrete improvement. They let the inspiration slip away, so Hashem brought them to Mara. He showed them that they had immediately sunk back to their previous level and that He does have the power to provide, as He performed the miracle with the water and the tree. And he gave them a series of mitzvos. he made it concrete this time.
no more "inspiration". Now you get inspired and you take something with you. You find a way to actually improve yourself and act on it.
That, maybe, is why we read more than the splitting of the sea and add the parsha of Mara. because it completes the splitting.