During my stay at Taketomi Island, I joined a guided tour to visit Iriomote Island and Yubu Island to take a Water-buffalo cart. Here’s a guide to the best island to join a jungle cruise. 
Jungle Cruise on Iriomote Island
90% of Iriomote Island is covered with rainforest and it’s very famous for the Iriomote cat. The jungle of Iriomote Island has various kinds of Mangrove trees to protect and foster sea life. The atmosphere of Iriomote Island was totally different from Taketomi Island. It’s covered with green and moisture. The rainfall of Iriomote Island is higher than other islands near Ishigaki Island. We went to see a 400-year-old Heritiera Littoralis tree in the middle of Nakama river (17.5 km) by a tour boat. The tour took about one hour.
Our tour guide told us there were only 2 traffic lights on Iriomote Island. One of them was constructed near their Elementary School for educational purposes. Even during day time, I didn’t see any pedestrians on Iriomote Island. Many islanders might drive a car for their transportation. About 200 Iriomote cats are still existing, however some of them are killed by car accidents every year. The guide emphasized not to drive too fast.
Yubu Island
After our jungle cruise, we went to Yubu Island. Yubu Island is located about 400 m away from Iriomote Island. The island’s circumference is 2.15 km and its height is just 1.5 m. It is covered with subtropical plants. In the middle of the 20th century, many people left Yubu Island for Iriomote Island after a typhoon hit, but one couple stayed and planted Subtropical plants. Now, the Water-buffalo cart and Yubu Island have become an important part of the tourism industry. 
Water-Buffalo Cart
The transportation to Yubu Island is only by Water-buffalo cart in the shallow sea and it takes 15 mins from the landing place. Our water-buffalo was a 5-year-old clever girl. When we visited there, the sea level was about one meter and many Water-buffalo struggled to go through it, but she went round the holes in the sea bed wisely and carried 10 people smoothly.
Some tourist groups could stay only for 20 mins due to the tour schedule, but one hour would be the best to see around. My favorite area was Manta’s beach on Yubu Island. It had a nice wide view. There is a cafe to have a nice coffee. The panorama opened up suddenly after emerging from the subtropical forests. 
The butterfly garden and Bougainvillea garden were astonishing. At the Butterfly garden, we could see Idea leuconoe liukiuensis, one of the biggest butterflies in Japan, and golden pupa.
Data:
Recommended transportation is to rent a car or to join a guided tour on Iriomote Island. I joined a tour from Ishigaki Island to visit 3 islands, but many choices are available from Ishigaki Island. The average cost of a rental car is about 5,000 yen a day.
Access:
There was no direct transportation from Taketomi Island to Iriomote Island, so I needed to go back to Ishigaki Island to take a ferry to get there. 40 mins from Ishigaki Island by Ferry. There are two main ports on Iriomote Island. One is Uehara Port in the west part of Iriomote Island. It tends to often be closed due to rough sea. The other is Ohara Port in the east part of Iriomote Island. If the boat to Uehara Port is cancelled, a temporary bus will be provided from Ohara Port to Uehara Port. From Ohara Port, it’s easy to access Nakama River and Yubu Island where you can take the Water-buffalo cart.
One point:
One or two day stay on Iriomote Island is recommended. I really wanted to stay longer for trekking in Iriomote’s jungle. The weather was so changeable, so rain gear is essential. In Summer, insect repellent is also a must-bring.
I really like to visit Japan someday!