It helps everyone feel lively, it adds variety, and so many types of music can be used. Rock for the aerobics, slow and mellow during the cooldown and stretch portions. One new trend I'v e noticed is that water massage is being used by the licensed massage therapists. And there are special water step classes. Far gentler on the knees than land.
Water has so many possibilities.
Water training and instructor certification is so crucial to having the best possible teachers. As for trends, I'm seeing people want to work most in the shallow end doing aerobic type workouts. It makes teaching easier. There is better intensity control when exercising with music. People love it.