You don't need to cover water in the microwave and in fact water when microwaved should not be covered as it can cause pressure to build up and explode in the microwave.
You can simply place a bowl, glass or cup of water in the microwave and heat it for a few minutes or so until it's hot enough.
When you microwave water what happens is the water can become super heated and when microwaved too long the water can boil vigorously and even explode.
Water in a microwave often boils faster in the microwave than on the stove so if you microwave water you should microwave it slowly in 1 to 2 minute intervals and keep an eye on it so the water doesn't explode in the microwave.
Once the water in the microwave has been heated you should slowly open the microwave door and stand to the side of it so that it doesn't potentially explode on you and burn you.
However if the water is not to it's boiling point then it's safe to simply open the microwave door and retrieve the water.
Putting a glass of water in the microwave does work and provides moisture to the foods you're heating.
As the glass of water heats up and gets hot enough it releases steam into the microwave which creates moisture for the food.
You can also heat a glass of water or a cup of water or other beverages in the microwave.
I will sometimes heat a ceramic mug of coffee in the microwave for around 30 seconds to a minute when my coffee gets cold.
I've been heating coffee and even tea in ceramic mugs for several years now without any issues.
I've also always added a glass or cup of water to the microwave when eating things that required moisture so they wouldn't dry out.
The glass of water in the microwave or cup of water in the microwave has always worked for me.
You can also add a damp paper towel wrapped around the food such as bread or pizza etc to keep the food moist and prevent it from drying out.