Koh means island in Thai. Koh Chang is the third largest island in Thailand and also one of the less visited. It is much less famous than Koh Samui and others, because of the lack of infrastructures and tourism is just less developped here.
To get to Koh Chang by air, you need to take a plane to Trat airport, then take a car to the ferry boat terminal and take a boat to the island. Depending on your hotel location, it could be an other 30 min drive.
I stayed at the Amari resort and I don't know if it was because of the low season, but I absolutely hated it. The food got boring really quickly, no free internet, changing "rules", etc. Not good if you stay longer than a couple of days.
Moving around by taxi on the island is pretty expensive, 50 THB per person, it may be more expensive if you want to get to the other side of the island. You can rent motorbikes but I don't know how much it costs and seeing how people drive there, it could be a little dangerous. Be sure of your driving skills and don't drive at night!
Depending on the days, the sea would be clean or filled with trash. This is sad but even though there are much less tourists in Koh Chang, the sea looks dirtier than on other islands! Hotel personel should pick up trash on their beach but at Amari, they didn't.
If you want to dive and observe things, you need to go further, but near the beaches, the water is not clear.
Despite the bad weather due to the rainy season, I still could enjoy pretty sunsets every evening :)