O porto fica cheio de barcos de pesca de madrugada.

In Portuguese, ficar often conveys a resulting state or change (to become, to end up).

• O porto fica cheio… suggests “the port ends up / becomes full.”
• O porto é cheio… would sound odd, as ser describes permanent characteristics.
• O porto está cheio… simply states a temporary condition, but without the nuance of “turning into” that ficar provides.

De madrugada refers to the very early hours of the day—typically from around midnight until sunrise.

• It is earlier than de manhã (which starts at sunrise and goes until noon).
• You might translate it as “in the pre-dawn hours” or “in the dead of night.”

After cheio, the standard preposition for expressing “filled with” is de:

• cheio de água, cheio de gente, cheio de barcos.
  Using com (“full with”) is possible in colloquial speech but less idiomatic.
  You never say cheio da here unless you mean “full of the” specific items:
• cheio dos barcos would imply those exact boats you already know about.

After cheio de we typically omit any article when we mean “lots of X in general”:

• cheio de turistas, not cheio de os turistas.
  Adding an article (dos barcos de pesca) would restrict the meaning to “full of those particular fishing boats you and I know about.”

Yes, but the nuance changes:

• de madrugada = between midnight and sunrise.
• ao amanhecer = around sunrise.
• de manhã = roughly from sunrise until noon.
  So choosing another phrase shifts the time window you’re describing.

Context and capitalization make the difference:

• Lowercase o porto means “the harbour.”
• Uppercase Porto is Portugal’s city (often preceded by o in speech: “Vou ao Porto”).
  Since our sentence starts O porto fica…, the lowercase porto points to the dock/harbour.

Adjectives in Portuguese agree in gender and number with the noun they modify.

• Porto is masculine, so you use cheio.
  If you said a doca fica cheia…, the adjective would turn feminine because a doca is feminine.