Free! ED (youtube) sent waves through the anime community when it first aired, with its bold decision to star the cast in what looks like an underwater SM dance club. But putting that aside, there are a couple of powerful symbolisms and references in the ED, which made this ED particularly enjoyable for me.
First of all, the general setting of the ED is in a desert. The most important thing in a desert is water, which is also the most important thing to a swimmer, which the characters are. In the middle of the ED, the characters are shown to be out of water, whereas the antagonist has an abundance of it and wastes them, symbolizing the tension between them. At the end of the ED, the MC comes across an oasis, where they proceed to swim in it, closing the symbolism loop of the importance of water for both desert and swimming.
In the dance sequence, the 4 characters take turns for their solo dance, and we see that their dance moves are unique and repetitive. This in fact symbolizes their swimming moves. In the show, the 4 of them are part of a swimming medley relay team, where they specialize in different swimming strokes. Equivalently, in the dance sequence, they have unique dance moves that resemble either their dance moves, personality or song lyrics.
Makoto swims the backstroke, while Haru has a nonchalant personality.