אז ישיר משה ובני ישראל את השירה הזאת לה:

The Midrash says it says they sang to Hashem to teach that they sang to Hashem and not to any human being - isn't that obvious?

Our focus is always on the victory and not on the war (e.g. Purim is commemorated on the day we celebrated not when the enemy was destroyed)

What are the חק ומשפט that were given at Marah?

The Chofetz Chaim on the lesson in patience we learn from the contrast between the lack of water at Marah and the abundance at the next stop (Eilim).