From Trat there are 4 ferry terminals to get you and your car to Koh Chang.

Laem Ngob to Ao Sapparot pier on Koh Chang is one choice and isn't somewhere where you would spend the afternoon sunbathing.
The Trat side is sign posted the Koh Chang Ferry Pier or the Tammachard Ferry

Laem Ngob to Than Mayom Pier on Koh Chang, here they have what is commanly refferred to as a thriving market, this I believe is a pedestrian ferry.

There are 2 centerpoint piers in Trat going to Centerpoint in Koh Chang.

All these 4 piers in Trat are quite close to each other, and the 2 main piers on Koh Chang are quite close, Centrepoint is the main one for cars.