In water distillation, the botanical material is completely immersed in water and then is boiled. This method protects the oil to be extracted till a certain degree since surrounding water prevents it from overheating.
Then it is condensed and cooled down. The oil is then separated out, as it layers on the top of the water. The water separated out in this process is termed as floral waters (also called hydrosol or sweet water) such as rose water, orange water and lavender water.
Water distillation can be operated at low pressures to reduce boiling temperature. This helps in protecting the botanical material and essential oil as well.
This method is used for extraction of oils that are sensitive to heat.
If a lot of exposure to hot water is not mentioned for a particular plant such as lavender, it is best to find an extraction method better suited.
Any botanical material that contains high amounts of esters do not take well to this extraction method, since the exposure to hot water will break down the esters into alcohols and carboxylic acids.