Ko Ngai (เกาะไหง) is an island district in Krabi Province, but actually it can better be reached via Trang in the Southern Andaman Coast. Like many other coastal destinations in Southern Thailand, it is known for its diving and long white beaches.
Koh Ngai (Hai) is a tiny island measuring 4 kilometres long and 2 kilometres wide. Mountainous and covered by tropical forest, much of the coastline is indented with rocky headlands, while the eastern part has some beautiful sand beaches, and in the south coconut plantations and more beautiful smaller beaches. The island has a white beach and clear water and is surrounded by magnificent corals.
Don't be surprised to see crab-eating monkeys and monitor lizards coming down from the mountains. In the uninhabited western and northern parts of the island, bird watchers should have a field day looking out for migrating birds. Snorkeling is easy along the western and southern coast, where some reefs can be accessed directly from the beach.
Ko Ngai or Ko Hai (เกาะไหง) is an island that is actually in Amphoe Ko Lanta of Krabi but it is classified as a tourist destination in the Trang Sea because it is easier to get there from Trang. From Pak Meng pier, located 15 kilometres away on the mainland opposite, in Trang Province. Standard ferry takes about one hour. A boat can be rented for 1,500 baht (10 people) and 6,000-10,000 baht (for 20-40 people). A faster speedboat connection from Pak Meng Pier is also available, departing around the same time. It takes just 20 minutes.
From Koh Lanta there is no scheduled ferry service to Ko Ngai although it is possible to rent a private longtail from Saladan (Ko Lanta) during high season (Nov - April). Another possibility is to buy an snokelling trip from Ko Lanta and leave boat at Ko Ngai.
There are no roads on the island, so getting around from one beach to another is done on foot, or longtail boats that can be hired at the resorts.
There is a foot path (jungle trek) from the main beach on the east to two beaches on the south, the westernmost of which has Koh Ngai Paradise resort. From there, you can follow a path that goes to National Park headquarters area and then to a hill with a two viewpoints.
An accurate map of the island can be found at OpenStreetMap. 
Enjoy the view from the main (east) beach to the carst outcrop islets.
Chateau Hill has great views from its uphill pool area.
Apart from the diving an snorkeling in the vacinity, the hills on Koh Ngai offer some spectacular jungle trekking amongst virgin jungles.
Take a kayak and see the island from the sea at a distance.
Most of the resorts offer trips to nearby islands for sightseeing and snorkeling. Emerald Cave on Koh Mook is one of the more popular attractions.
All the resorts have restaurants that serve fresh seafood, Thai and semi-western dishes.
Thapwarin Resort has a notably good restaurant with friendly staff and high quality cuisine with wide selection of dishes.
Fantasy Resort boasts an impressive-looking restaurant.
All resort restaurants offer beer, many of them offer wine as well.
CoCo Cottage has a bar with a long selection of cocktails.
Accommodation is available on the island.