The simple answer is that there are literally thousands of chemical reactions (mediated by enzymes) going on in the cell. Some produce H2O, others consume it. For example, when proteins are broken down into their individual amino acids as part of normal turnover in the cell, a water molecule is necessary to break the peptide bond. Therefore, the overall balance of water produced vs. water consumed is fairly equal, especially when you take into consideration the consumption of water and its excretion via urine and sweat.