Water Addition
The oxygen of water is nucleophilic enough to attack a carbonyl group. However, the reaction is of little relevance because the hydrate formed is less stable than the starting aldehyde or ketone.
The acidic medium protonates some aldehyde or ketone molecules. The protonated molecules are more electrophilic and can be attacked by a neutral water molecule. These equilibria are also generally shifted to the left.
Hydroxide, acting as a base, removes an α hydrogen and gives rise to the aldol condensation.
The relative instability of the hydrates makes this reaction irrelevant.
If you are curious, click here to visit the aldol condensation in advance.