does the swim activity work for water aerobics?

Hi

No - the Workout app includes two swimming activity types: Pool Swim and Open Water Swim. However, both of these activity types assume that you will be swimming continuously, so neither is appropriate for water aerobics / fitness.

For water fitness classes, instead record your activity as "Other". This will credit one minute of Exercise for each full minute of the workout. Active calories will be estimated at a rate equivalent to a brisk walk or based on data recorded by the heart rate sensor, whichever is higher.

When saving your workout, tap on "Name Workout" to label your activity as Water Fitness.

You may also wish to lock your watch to prevent accidental screen interactions during your workout:

• About Apple Watch water resistance - Apple Support
• Work out with your Apple Watch - Apple Support