Make sure to turn off background app refresh and check walkie talkie is not accidentally turned on.
Check there is no workout running (yellow circle at top if screen.)
Make sure you have no VPN or VPN based ad-blocker on the iphone. Uninstall it then remove any profiles (iPhone Settings, General VPN & Device Management)
Force restart your watch i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.
Then while the Watch is restarting Force Restart the paired iphone Force restart iPhone – Apple Support (UK)
Try it for a couple of days especially if you sync photos to your Watch.
If the battery still drains then Unpair the watch and then pair it again.
Unpair and erase your Apple Watch – Apple Support (UK)
Set up your Apple Watch – Apple Support (UK)
More info
Apple Watch - Battery - Apple (UK)
Batteries - Maximising Performance - Apple (UK)
If it still discharges after all Watch resets try this user tip
Apple Watch battery drain - the ultimate … - Apple Community