Half an hour from Phuket and a world away from it. Rice fields, rubber trees, limestone on the horizon and no nightlife at all.
Find a room before you go.
Koh Yao Noi lies in the middle of Phang Nga Bay, with Phuket to the west and Krabi to the east, and it has resisted becoming either. Most people here fish or farm rubber, the island is predominantly Muslim, and there is no bar street, no fire show and very little to do after nine in the evening.

That is the point of it. What you get instead is a quiet road through rice fields and rubber plantations, water buffalo standing in the wet season paddies, and a horizon full of the limestone towers that everyone else pays for a day trip to see. Dress modestly away from the beach, and expect alcohol to be available in tourist places rather than everywhere.
Affiliate link. If you book through it we earn a commission, at no extra cost to you.
Almost all the accommodation is strung along the east coast, facing the bay and the sunrise. Tha Khao in the north is the old fishing village and the arrival point for boats from Krabi; Hat Pasai in the middle is the most popular stretch of sand, with a handful of small resorts and beach restaurants.
Klong Jark further south is quieter again, with a couple of the island's nicer places to stay and a genuinely undisturbed beach. Anywhere along here you wake to karst islands out of your window, which is what people come for.

Koh Yao Noi's beaches are shallow and heavily tidal. At low water the sea can retreat several hundred metres and leave flats you walk out across; a few hours later it is back. If you want to swim on demand, book somewhere with a pool, and check the tide before you plan a beach afternoon.
This is not a flaw so much as the character of a bay island. The trade-off is that the water inside Phang Nga Bay is sheltered — calmer and more usable in the monsoon months than the open west coast of Phuket, and perfect for kayaking out among the limestone.

If you want Phang Nga Bay without Phuket's traffic or Krabi's crowds, very much so. If you want nightlife or a long swimming beach, choose elsewhere.
Yes, but around the tide. The east coast beaches go far out at low water, so most visitors swim at high tide or use a hotel pool.
By boat from Bang Rong pier on Phuket, about half an hour, or from the Krabi side near Ao Thalane. Both run several times a day.
In tourist restaurants and resorts, yes. It is a Muslim island, so it is not sold everywhere and there is no bar scene.
More Koh Yao Noi and Andaman coast guides, plus the rest of Thailand's islands.