Yeah my buddy who does outdoor construction thought he was fine just chugging a couple bottles at lunch. Last summer he nearly passed out tying rebar, said his vision went all spotty and his knees just buckled. Turns out he was working in full sun for hours with basically no water in his system. The scary part was his pee was dark like tea even after that, doctor said he was way behind. Now he carries one of those big gallon jugs and forces himself to drink even when he doesn't feel like it.