Activity Tracking: Recovery score, strain score, heart rate, active minutes, workout modes, calories, temperature, hydration, workouts. Alerts you to move if inactive.
Notifications: Displays calls and messages.
Water resistant: Yes

Activity Tracking: Monitors steps, calories, distance, heart rate, sleep, GPS, blood oxygen, fall detection. Supports a variety of workouts like running, cycling, yoga and swimming.
Temperature Sensor: Added on Series 8 and 9
Notifications: Displays notifications from your phone.
Water Resistant: Yes