The main beach and the pier are on the east side and there is a small beach with the sweet name of ''Paradise Beach'' on the other side of the island. I did not go so I can not tell you if it is worth to go but the beach name is lovely.
If you are looking to book a room in Koh Ngai, you will often find Koh Ngai Resort but it is located at the pier on a stretch of beach not very nice. I recommend you to go to the other resorts on the large beach next door. A small steep path connects the two beaches and longtail boats regularly shuttle between the pier and the large beach (50 THB per person).
Hotel IN_WORD Koh Ngai
Homestay IN_WORD Nong Khai
The best time to come to Koh Ngai is from December to March knowing that during the low season from May to the end of October, when it rains the most, tourism activity is very limited and most of the resorts on Koh Ngai and other Trang islands are closed.
From May to the end of October, in low season, I don't advise you to go to Koh Ngai, the weather can be very rainy and most of the resorts are closed.