It has good intentions, but Manaphy was designed poorly. If Manaphy could knocked itself out with it's ability to search for a card, rather than the opponent knocking it out, then I would really enjoy playing this card. For example, while Manaphy is your Active Pokemon, use Seafearing, get some energies on the bench. Then, during your next turn, knock Manaphy out, promote the Pokemon you placed energies on, and further support the new Pokemon to setup with whatever card you search for (Eviolite, Rare Candy, Energy Switch, etc).
As it is, though, I'm not a fan of Manaphy. Too many factors outside of your control. It's ability relies on your opponent, and it's attack relies on coin flips. Final Wish can be avoided with poison damage, and while Seafearing is a decent attack, it requires Manaphy to be your Active Pokemon, with an energy on it, and enough energies in your discard pile. This all too hard to setup during the early game, which is when you would want to use Manaphy.
5/10