Based on thousands of users' experience and how many apps are running at the same time, you should be able to track activities continuously for 5 hours on Apple Watch 4 and more on higher versions (following the below suggestions to save battery). It is usually enough for tracking a Water Sports day.

If you need to get the maximum from the battery life you should set your Waterspeed on Apple Watch as:

• Dark mode activated (on iPhone)
• 'Save Workout' deactivated (it won’t record HR and Calories)
• 'Distance alarm' deactivated
• 'Sounds on start'
• Chose Other, as sport, to stop recording Paddling movements

You can also see here some advice for the general settings of your watch: https://ios.gadgethacks.com/how-to/13-ways-extend-save-battery-life-your-apple-watch-0161656/