Hamachi is just the Japanese name for yellowtail. Quite possibly my favorite when it comes to sashimi...although on any given day, I might have a more potent craving for the medium and extra fatty toro (tuna) that they serve at Karuma Zushi.
As for Arctic Char, it has some characteristics of salmon, but is also milder in flavor and more delicate in texture (I could be biased by Aquavit's perfect rendition). For more information on the matter, I would ask Tom Armitage. He cleared up a kampachi confusion a while back.