Complex sentence: Salinity varies widely, being lower near the surface and the mouths of large rivers and higher in the depths of the ocean; however the "relative" proportions of dissolved salts vary little across the oceans.
Split into simpler sentences: Salinity varies widely, being lower near the surface and at the mouths of large rivers while being higher in the depths of the ocean. The "relative" proportions of dissolved salts, however, vary little across the oceans.