Use Hammer Shark's effect to special summon a WATER monster from your hand. Then use Aquamirror Cycle to bring back the special summoned card back to the deck to get 2 WATER monsters from your Graveyard back to your hand. On the next turn, you can normal summon 1 of those cards and special summon the other one by again using Hammer Shark's effect.
The effect returning a monster on your field to the deck and getting monsters from the graveyard are resolved at same time.
A set Water-attribute monster cannot be targeted by the effect of this card.