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The safe answer is that you should never use your cordless drill in the rain or around water. At the same time, even when you've taken measures to protect your cordless drill from water, it's best to keep the water around your drill to a minimum.
Hereof, can you use a cordless drill while standing in water?
There is no danger in using a cordless drill in water.
In this regard, can you use power tools in the rain?
Do not use electric tools in wet conditions or damp locations unless tool is connected to a ground fault circuit interrupter (GFCI). Do not expose electric power tools to rain or wet conditions; wet tools increase the likelihood of electric shock.
Kind of like Never Wet or Liquipel the spray that you can get for your electronic devices to keep them waterproof. They don't claim the batteries to be waterproof, but it is a huge help in damp conditions. They also announced what I like to call the Super Charger. It holds 3 M18 and 3 M12 batteries.