Yes, there is a risk. Even a properly installed and protected electrical system can fail to protect you, either because of unforeseen situations or component failure.
There are two types of flaps.
One is watertight only when not in use and closed. This is probably what you have. This type of outlet is only meant to be used temporarily when exposure to weather is not a problem. For example, plugging in a leaf blower. This type of outlet should not be used for holiday decorations.
The other type is watertight even when you have something plugged in to it, provided you close the cover.
Fortunately, changing from one to the other is pretty easy: just take the cover off and put the other one on, making sure to follow the directions to maintain watertightness.