Not only did his rapidly expanding sand field catch Bocchiere in time for the movement into a kata (considering he had only stopped instead of backed off after all, and the spread was accelerated by the broken up spheres), he had also simply opened eyes too. Just as aforementioned the chakra was indeed everywhere, so even if he hadn't caught the movement then due to his doujutsu even the tiniest beginning of a shift in the power present would have made it clear something was coming.
After all he had just let it drift about before then on its own, so what else could it have been but something harmful? As destructive as even lone drops of water could be however, it'd take a moment (even if ever so tiny) to accelerate it to damaging velocity, which coupled with the warning he'd get via sand and sight would leave him enough time to counter.
The rather minute amounts of breathed in mist were neutralized with him hardly needing to do anything, for so strong was his lean towards fire he was the equivalent of a walking furnace, able to literally spit flame with nary a seal formed. Needless to say, even if any of the tiny amounts of water natured chakra (tis mist we're talking about after all) would have avoided eradication from contact with his inside, it'd have been completely erased by what he actually did to counter the jutsu.
While some preferred to form a seal while molding chakra for sake of focus, Warren hardly needed to do such a thing at his level. This too would give him another minor speed advantage over Bocchiere who did require one for his jutsu, for all he had to do was will it without moving a muscle. Bursting out like a nova, his fire natured power flared so explosively that not only would it completely purge any intruding elements that may have yet remained inside him, but also strike a blackened circle to the ground around his feet with licks of flame left behind, not even to speak of completely blowing apart any Bocchi's chakra in his vicinity.
Furthermore channeled along his sand, rinnegan 'material' being quite conductive of its creators power after all, although it wouldn't quite entirely erase the mist since it extended beyond his sands current reach it would still punch quite a clean big hole in its center. This'd leave both combatants visible, lest Bocchi chose to dive back into the mist of course.
Edit: added forgotten detail of sand spread speed