The speaker spends most of the poem watching someone they clearly adore. The repeated "I watched you" works like a refrain, almost like someone replaying a favorite memory. We see the person by the water: dancing, smiling, toes at the edge of the shore, polka-dot legs. The small, affectionate details suggest a crush, and the line about legs "longer than class without you" is a funny, honest way of saying that time drags when they're not around.
The mood shifts in the last stanza. The speaker finally tries to join them, and for a moment there's warmth in the shoulder. But the waves are cold, and the speaker ends up back where they started, standing back and watching. The ocean seems to stand for the gap between the two people, or for the risk of getting close. The poem feels tender and shy at first, then wistful, with a quiet ache of longing that doesn't quite get answered.