Hi Jim,
Although there is not a specific category for water aerobics, you can add the data manually into the Health app on your iPhone. There is a "Water Fitness" category in the Health Data > Fitness section.
Collect data from Apple Watch. Once you pair Apple Watch with your iPhone, data is automatically sent to the Health app. For example, to see heart rate data recorded by Apple Watch, open the Health app on iPhone, then tap Health Data > Vitals > Heart Rate.
Collect data from another device. Follow the instructions that can come with the device to set it up. If it’s a Bluetooth device, you need to pair it with iPhone—see Bluetooth devices.
Collect data from an app. Follow the instructions that can come with the app to set it up, then watch for a sharing request where you control whether data is shared with the Health app.
Enter your own data. If the data type item is in your Dashboard, just tap it there, then tap Add Data Point. Otherwise, tap Health Data at the bottom of the screen, tap the item you want to update, then tap Add Data Point.
Stop app data collection. Tap Sources at the bottom of the Health screen, then select the app in the Apps list. Or tap the associated data type item in your Dashboard or in the Health Data list, tap Share Data, then choose the app under Data Sources.
Take care,