I am going to try the following which I have paraphrased from this discussion: Tracking water walking with Apple Watch Series 2
When recording workouts via the Workout app, choose the activity type that most closely matches your workout. This enables your watch to use the most appropriate sensors and data sources when estimating results, including using tailored calorie calculations for certain activities.
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 using a recognised swimming stroke, so neither is appropriate for water walking.
Indoor Walk is also not appropriate, as the Workout app does not expect you to be walking in water. When expending the same amount of effort, walking in water will be at a significantly slower pace (due to the increased resistance) than walking on land. As a result, recorded results would be inaccurate. You may, for example, receive little or no Exercise credit, due to the watch believing that the intensity of your activity is not equivalent to or greater than a "brisk walk".
For water walking, instead record your activity as "Other". This will credit one minute of Exercise for each full minute of your 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. For convenience, the workouts that you do most often will be offered as Quick Start options within the Workout app (including previous Other workouts with their saved labels pre-selected).
I still strongly advise you lock your watch using Water Lock to prevent accidental screen interactions during your workout. This will also expel any water from the speaker when turning off the lock:
