The Latin music plays for an hour as the class of 14 women dance their way through samba, merengue, and salsa beats in the pool at the Waltham YMCA.

Instructor Justin Falxa choreographs the moves for Aqua Zumba to match each song, just as he does for the standard Zumba classes he teaches, but as he demonstrates the dances on the pool deck, he stretches the movements out and slows them down to mimic the resistance of the water.