Learn Turkish: Ördekler suda yüzüyor. - The ducks are swimming in the water.

In Turkish, when the subject is plural but not human (like animals, objects, or concepts), the verb usually stays in the 3rd person singular form. Since ducks are animals, we just use yüzüyor.

Since we are learning gibi (like), how would I say "swimming like a duck"?

You place gibi immediately after the noun you are comparing to. To say "He is swimming like a duck," you would say (O) bir ördek gibi yüzüyor.

What does the -da suffix do in suda?

The -da suffix is the locative case, which translates to "in," "on," or "at." Here, attaching it to su (water) gives us suda, meaning "in the water."

How do we know to use -üyor for the present continuous suffix on yüz- (swim)?

The base present continuous suffix is -yor, but it needs a connecting vowel after the consonant in yüz-. Under 4-way vowel harmony, the vowel ü in the stem dictates that the connecting vowel must also be ü, giving us yüzüyor.