When Cloud stands waist-deep in the water, surrounded by splashing children, he scans the delighted crowd and catches Tifa's eye.
She smiles; he does the same.
The last time she had seen his smile, she had thought everything was all right only to be crushed when he did not return from a job. It marked the start of a painful period of worry, stress, and agony in every sense of the word. Now both smiles are full and honest; there is no doubt to mask.
With their smiles, Tifa finally gets her happy ending.