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.
Also strength training is big, using special paddles or HydroTone equipment. One of the most popular types of class is where students are encouraged to talk, enjoy the other students, have fun, and know that they feel better because of going to the class. These of course would attract very different age groups. Find a teacher, certified, who can handle a multi-ability class. Their knowledge and experience enable them to adjust equipment use and intensity of the workout so that each student can proceed at his or her own level and pace.