The castaways talk into the mouths of the fish Gilligan just caught on the possibility one of them swallowed the radio transmitter. Mr. Howell's voice (reading stock market reports) is clearly heard well before Mr. Howell comes onto the scene and joins the other castaways' efforts.
The flashback scenes show the original 3 cast members from the "Lost Pilot" Gilligan's Island: Marooned. Ginger has brown hair, the Professor is actually a Teacher (played by a different actor) and Mary Ann is "Bunny," a blonde. All 3 actors/characters (except Ginger although she was changed to a redhead) were replaced after the Lost Pilot as the series was almost a year later. However, the flashback scenes show these 3 characters when they are sleeping on the boat. Also when Gilligan is fishing and casts the transmitter and radio into the ocean you can tell that the Professor is actually the Teacher from the Pilot. His hair is shorter and he is wearing different clothes than the Professor.