The only obtainable method (I know) of getting Kingdra without Wi-fi is to trade with a friend locally. If you've got a friend, simply trade your Seadra holding its Dragon Scale with your friend for their pokemon, and then trade back.
Unfortunately, in HGSS, there is no means of obtaining Kingdra in-game without trading. (To my knowledge and research.) Sorry. Although there is a variety of other great water types available for capture In-Game throughout HGSS, so why not try one of those?