Hi
Apple Watch Series 2 and Series 3 models are suitable for shallow water activities, such as shallow water swimming.
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 or pool pilates.
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 wish to lock your watch to prevent accidental screen interactions during your workout:
For the best performance from the heart rate sensor (the accuracy of which can potentially be affected by water), be sure to wear your Apple Watch snugly (but not too tightly) during your workouts. Apple suggests tightening your Apple Watch band before workouts and loosening it again afterwards:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support