A big island with almost nothing on it. Where you book decides not just your beach but, on many nights, where you eat.
Find a room before you go.
Koh Kood is the fourth-largest island in Thailand and one of the least developed. There is no resort town, no strip of bars, no ring road full of shops — just one hilly concrete road down the west side, a few small villages, and resorts spread thinly along the bays between them.

That has a practical consequence people miss when booking. On most islands you pick a beach and then wander out for dinner. Here, on quiet stretches, your resort's restaurant may be the only one within walking distance, so it is worth checking what is nearby before you book — and worth renting a scooter if you want a choice.
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Ao Klong Chao is the closest Koh Kood gets to a centre: a wide beach with a river running out behind it, a handful of restaurants and shops, and enough places to stay at different prices. You can take a longtail or a kayak up the klong to the waterfall, which is the island's set-piece excursion.
Ao Tapao and Ao Ngamkho just south are smaller and quieter versions of the same, with a few good mid-range resorts. Together with Klong Chao they hold most of the island's affordable rooms, and they are the sensible base for a first visit.

North of Klong Chao the coast opens into long, near-empty beaches, and south past Ao Phrao the road climbs and the bays get smaller and more remote. Ao Bang Bao in the south is one of the prettiest on the island, with only a couple of places on it.
Over on the east side, Ao Yai is not a beach resort at all but a fishing village of wooden houses on stilts over the water, with seafood restaurants and boats. It is worth an afternoon even if you sleep elsewhere. Everywhere at these ends, distances are real and eating out means driving.

Klong Chao for a first visit: it has the widest choice of rooms and restaurants and the waterfall behind it. For quiet, go north or to Ao Bang Bao in the south.
Yes — calm, shallow water and no traffic to speak of. Choose a bay with more than one restaurant unless you are happy eating in every night.
Essentially none. A handful of beach bars close early. This is the island people choose precisely because nothing happens on it.
By speedboat or ferry from Laem Sok pier near Trat, around an hour. Trat has a small airport with flights from Bangkok.
More Koh Kood and eastern Gulf guides, plus the rest of Thailand's islands.