Symbolism, Imagery, Allegory
Question: How do we know that Meursault is not, in fact, a robot? Answer: Robots can't swim.
Laughing and swimming are the two actions that remind us that, in fact, Meursault actually is human being after all. Meursault is also strangely attracted to laughter (this isn't strange for normal people, but Meursault is not normal). He finds Raymond instantly friendly... once the man laughs. His heart melts and he wants Marie—in a sexy, sexy way—each time she laughs. For Meursault perhaps, laughter symbolizes innocence, simplicity, and nonchalance.
What is true of laughter is also true for swimming. Meursault loves to swim. Marie adores swimming. The two meet while they were swimming, and Meursault essentially makes his first move while in the water. Carefree, wholehearted goodness? You bet. Too bad it doesn't last.