"Hey… What do you plan to…," John hadn't completed his words when Will's sea cannon opened fire.
The sea cannon dealt extra damage to structures. It managed to blow a hole in the upper deck's floor. It did the same here. The wall at the end of the corridor was shattered by the sea cannon shot. That wall bordered directly the sea and this deck's position was below the sea level.
Under normal circumstances, the seawater from outside would have rushed in through the hole and pushed everyone back into the corridor. However, Will's Roaring Ocean was still in effect. The magical pressure from this skill was stronger than the sea outside, so everyone was pushed out through the hole instead.
Before they knew it, everyone found themselves underwater outside.