Will does all of his deep free dives using only eye-covering swim goggles. All depth diving must be done with a scuba mask that covers your nose to allow for mask pressure equalization. Without equalizing your mask, at even 10m, the mask squeeze causes your eyes to feel like they will burst from your head. Beyond 10m can be injurious. Mask equalization is mandatory on all such dives.
When Zane free dives and his buddies show up to help him cheat, there are total three divers in the water breathing oxygen. The resulting bubbles would be clearly visible at the surface, making his ruse known to Will and Sophie. Instead, there are no bubbles seen at the surface.