"Skipper" is a naval term, not a rank, and refers to a person in charge of a ship, boat, watercraft, vessel or shore installation. In the US Navy, larger vessels are frequently commanded by a captain (0-6). In the U.S. Marine Corps a captain (0-3) may be referred to as "Skipper" as a term of endearment.
In this episode the name/term "Skipper" is mentioned by the castaways, and in the theme song/closing song) a total of 60 times. The only castaway who does not say Skipper is Ginger (Tina Louise).
This is the third time a gorilla is seen on the island. The first time was in Diamonds Are an Ape's Best Friend (1965), the second was Beauty Is as Beauty Does (1965), then this episode, the fourth was Forward March (1966), and the fifth was Our Vines Have Tender Apes (1967). There has to be at least two, as one is a female.