he has more than 1 deck so that's probably all it meant. he only really used water dragon + electromagnetic warriors but those are anime-only cards. op shows him with a 'fire dragon' counterpart to water dragon but it was never shown once in that anime
His boss monster probably varies depending on which deck he uses. Water Dragon's his most prominent, since most of his decks (aside from his EARTH one) borrow heavily from the WATER deck. But Conduction Warrior Linear Magnum ± is clearly the boss of his EARTH deck. His boss in the manga is Red Ogre.