To get a Kingdra you must first obtain a dragon scale. To get a dragon scale, you have to either catch a Horsea or a Bagon. There is a 5% chance that they will be holding a dragon scale. Trade your seadra when it is holding the dragon scale, and it will evolve into the best pokemon that is not a God, Kingdra.
Please take note: This takes time. I could 174 horseas and 291 bagons before I found a Dragon scale. However, it's definitely worth it!
Hope this helps.