Before they departed, Henry gave everybody a crystal that would allow them to be able to move and breath within the water. With this water adaptation crystal, they would be able to maneuver within the water as if they were on land.
Since the majority of them could fly, that implied that moving in water would not be a problem at all. And with that, the disadvantage that they would have gained within the water was completely eliminated.
The moment that everybody was given the water adaptation crystal, they could not help but get excited. It was also at this moment that they finally remember the fact that they were actually going to fight within the water, and not on land.
