You might show the incantation in full the first time the character uses it in that issue of the comic book.
And the next time the character uses it the word balloon might say "O water, garden..." and the next panel might show a water surge happening and a word balloon from outside the panel with "...Child of Gaia! Aqua!"
And maybe the third time the character uses it in the comic book the character starts saying "O water, garden of life..." and in the next panel the water surge happens and the readers assume the character completed the spell.
And if the water surge is some sort of weapon, you can sometimes show the character starting the spell "O water, garden of____" and being hit by an enemy attack, stopping them from completing the spell. This might happen several times before they can complete the spell and use the water surge.
And maybe you can have the character start to say "O water, garden of...*" with a note at the bottom of the panel or at the bottom of the page: *The complete spell is: "O water, garden of the soul, spirit of the moon, hallowed source of life, I command you to converge before me, I, child of Gaia! Aqua!"
And you should probably show the complete spell at least once in every issue that the character uses it.