Many words in English are based on the same root words, and knowing what these mean can help us with spellings.
The root word 'hydro' (sometimes spelt 'hydra') comes from the Greek word for water, and it forms part of a lot of words in English.
For example:
The gas that combines with oxygen to make water is called hydrogen.
Someone who feels unwell because they haven't drunk enough water is said to be dehydrated.