Water is the enemy of all construction. Water causes rot, rust,
delaminating, erosion, etc. Bathrooms and kitchens are a particularly bad
place because you have lots of water around and some of it leaks. The first
step after finding water damage is to figure out how to prevent future
damage. Do this even before you repair existing damage as it may affect
your repair strategy.
In any building spend a lot of time trying to figure where water can get in
and how to prevent it.