"We have to precisely know the wave action."
Translation:On doit connaître exactement l'action des vagues.
I think it's "l'action des vagues" and it usually refers to "wave action" in a nautical sense. For example, "L'action des vagues peut mettre en danger un petit bateau."
I'm not sure why this English sentence couldn't be referring to the action of a single wave though, a tsunami for example.