If you don't mind staking out your spot about one hour before Illuminations starts, find a place on the bridge just to your right, after entering EPCOT from the International Gateway. When standing on the bridge, above the canal entrance, you will see the fireworks cruise boats park right below you, starting about 10-15 before showtime. There are usually quite a few that pull up there.
Basically, watching from the bridge is the same view, at no extra cost! The major difference is that guests on the boats can sit and enjoy their snacks, while watching the show.
The time when it might be worth booking the cruise would be during those super busy, peak crowd times, such as Spring Break, Winter Break, or on holidays.
If you don't want the extra expense for the cruise, there should not be any problem finding a good spot; anywhere on the bridge will give you a good view.