Hello. I know that the aw2 has a swimming activity that will automatically seal the watch so you can eject the water later. But what about things like when it's pouring down with rain, do you have to take precautions and seal it manually?
It doesn't seal the watch. The ejector simply ejects water from the speaker by vibration.
You don't have to "lock" the watch before getting it wet. You can lock it via Control Center, or it will automatically lock during a swim workout.
If you didn't lock it before getting it wet, haven't done a workout, and the speaker sounds funny after getting wet - you can simply toggle the lock on/off in Control Center.
The lock part is that the screen locks from touch input so water doesn't cause false inputs (if you've ever tried to type something on an iPhone in the rain you'll know what I mean). You don't need to lock your watch in order to prevent water damage though.