Sadly, most visitors to the city experience only a tiny section of the ferry route, shuttling between Saphan Taksin pier in the south and the Temple of the Emerald Buddha or Khao San Road in the north (there are piers close to both).
However, if you catch the first northbound ferry of the day (which leaves Saphan Taksin at about 4:05pm), you will have enough time to reach Pakret, hop on the ferry to Ko Kret, explore the island and make it back to Bangkok in time for dinner.