I've actually made this deck myself, I use the eevolutions to give my gyarados type coverage. Any Pokemon that can use Double Colorless Energy is also not half bad either: Seismatoad, Mewtwo EX (expanded), come to mind.
Teammates is also a great card because you are constantly going to be losing gyarados and magikarp if your opponent gets set up faster, so Teammates searching any 2 cards from your deck is going to help. I can also not stress enough how important x4 Battle Compressor combined with Buddy-Buddy Rescue is for the deck. You just dump any Pokemon that you need to set up into your discard to just bring back to your hand, at the same time it recycles dead magikarps.