Not much is known about the island's geography other than the fact that the island is surrounded by weird, rough currents, which left Ace and Deuce stranded on the island with hardly any way to leave.
The island has trees on it, as a branch was used by Deuce in an attempt to kill Ace, and several others were crafted by him into a makeshift raft powered by Ace's newly obtained Devil Fruit, the Mera Mera no Mi.
Two years before the start of the series, Portgas D. Ace and Masked Deuce were stranded on Sixis with no way of escaping. The two talked, and Deuce discovered about Ace's heritage, causing him to distance himself from the pirate. Some time afterward, unable to find any food or water, Deuce saw Ace with a piece of fruit and decided to kill him for it. However, his plan failed and to his surprise Ace offered him half of the fruit, fixing their relationship. Upon discovery of fruit's nature, the two built a raft to escape the island.
Being a deserted island, Sixis has little to no wildlife. When stranded there, it was stated that Deuce nearly starved to death, driving him to consider murder for single piece of fruit.