Since Koh Chang is indeed a water-locked island, there would probably only be two logical ways to get onto the island, by air or water.
Getting into Koh Chang really only has the watercraft as an option, since the air routes generally tend to serve mainland Thailand, with no practicality in flights going directly into the surrounding islands, including Koh Chang.
If you are really lucky, you can hitch a ride on one of the fishermen's boats, but they rarely ever make the entire trip from the mainland to any of the islands so, one of the commercial options is the next best, and probably the only other option.
There is heavy traffic between mainland Thailand and Koh Chang, in both directions, so there will be no shortage of transport, particularly aimed at the tourist market.
You can pretty much get transport to and from Koh Chang all day, everyday, but it's better to travel in larger groups as some group discounts can be made available, rather than renting your own, private speedboat, which comes with a driver of course.
Renting the speed boat service is only viable if you are really in a hurry to get into or out of Koh Chang, but you must naturally be prepared to spend a little bit more than what you'd otherwise spend on the larger commercial water transport options, like the ferries.
Once on the island, there are various transport systems to choose from, from walking (traveling by foot is best if covering short distances while getting some sightseeing done) to bicycles for rent, which you can use the whole day for no more than $2, or you can take some of the bus services, taxis or motorcycles for rent.
It all depends on where you want to get to, how far it is and how much of a hurry you might be in.