A duck is swimming on the water.

Part of the duck’s body is in the water, so we use {in}.

A duck is swimming in the water.

She admire to the chicken.

The usage of {admire}: admire sth.
{She} is the third person singular, so please add an -s to {admire}.

She admires the chicken.

The cat is despise to its cat food.

{Despise} is a verb. The usage of {despise}: despise sth.
{The cat} is the third person singular, so please add an -s to {despise}.

The cat despises its cat food.