If you're talking about fireworks at DisneySea at the Tokyo Disney Resort, I'd have to say you probably could see little to nothing of the show from a Fantasy Springs room. First, the show is on the other side of the park from FS. Second, it's really more music, lasers, and projections than it is fireworks - it's not like fireworks at the Magic Kingdom at WDW. It's a great show and they do a nice job using the water and the buildings for the show, but you probably need to be closer.
Having said that, we both stayed at Fantasy Springs a couple nights, and we actually saw the night show while standing in the area near Miracosta, so I cannot say 100% there's no view from FS. But I don't think you'd see much, if anything. I'm not certain you'd even hear it.