A dipeptide bond is formed when two amino acids are linked together. As shown in the picture attached, when two amino acids are joined by a peptide bond, a H and OH ion atoms are lost from each of the two amino acids. Thus, for every 2 amino acids, 1 atom of water molecule is liberated. Thus, the total amount of water molecules liberated by 200 amino acids is 100.
200 amino acids * 1 water molecule/2 amino acids = 100 water molecules