How do i bash the barrier underwater in Poisiden's water in mythology island?

You will need Hercules to move the large rocks blocking the throne rooms of Poseidon and Hades. Hercules will not help you until you have collected all 5 of the sacred items and returned them to the Tree of Immortality (where Zeus steals them from you).

To transport Hercules, you will need the Touchscreen Mirror from Aphrodite's test on the beach (the gods are Ares, Hermes, Aphrodite, Artemis, and Hestia).