Explicit solvent all-atom molecular dynamics simulations of mixtures of poly(styrenesulfonate)-(PSS) and poly(diallyldimethylammonium) (PDADMA) polyelectrolytes at various salt (NaCl) concentrations are performed. We characterize the formed polyelectrolyte complexes (PECs) and relate the observed physical properties of PSS-PDADMA PECs to the properties found in polyelectrolyte multilayers (PEMs) made of the same compositions. Our results reveal a change in the way charges are compensated upon the addition of salt, namely from an intrinsic mechanism (polyanions pair with polycations) toward an extrinsic one (polyions pair with salt ions). The probability of the intrinsic compensating mechanism decreases from about 90% to about 60% when the salt concentration increases from 0.168 to 1 mol/L. The interaction energies of the ion-pairing follow the order of Na-Cl > PSS-Na > PDADMA-Cl congruent to PSS-PDADMA. Furthermore, we investigate thoroughly the water distribution and study the hydration mechanisms in our system. Water is found to be homogeneously distributed inside our investigated systems, while we find a negligible difference between the hydration ability of (PDADMA + Cl-) and (PSS + Na+). This lack of asymmetric behavior demonstrates that the observed swelling-shrinking switch during the buildup of PEMs cannot be related to the hydration behavior, and we suggest that the presence of a substrate has to play a critical role. A further analysis of the water structure shows that the dielectric constant inside such mixtures is roughly 1 order of magnitude lower than in bulk water, and our determined values compare favorably with experimental measurements. Finally the diffusion of water molecules inside the PE mixtures is found to be 2 orders of magnitude slower than that in pure water.