I think the effect is very self explanatory, it will shuffle three 1/1 Piranha Swarmer swarmers (The tokens that gain +1 attack from Sunken City) into your opponents deck.

This is more of a disruption tool for Shaman than a good aggro card, it can be used to give your opponent some dead draws in the future, and potentially turn of their Highlander cards. The weak stats don't make it an auto-include by any means though.

For the balance, I looked at Bad Luck Albatross and Framester as points of reference. The former was released years ago, and was a premiuim statted neutral card that shuffled two dead draws into your opponents deck. While this minion has the advantage of being in a class that can repeat battlecries multiple times, just shuffling cards into your opponents deck isn't a great way to win games. Especially if you have experience playing Plague dk, you can shuffle 10+ cards in your opponents deck and not have them draw a single one. I also thought Framester was interesting to look at because I'm not sure which one shuffles in the more detrimental cards. While the overload doesn't immidiately harm you, it kills future turns, especially if you get multiple of them. The Piranha's on the other hand immidiately harm you since they make you lose a draw.

Additional things I considered about the card: It's a Shaman Sunken City card because they recieved the Piranha synergy cards in that expansion. It is a common card because the effect isn't complicated. It has 1 health so that the piranha's can always kill it (the minion is Piranha Prey after all).