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For more on marking an answer as the "Best Answer", please visit our FAQ.Whales do drink some sea-water with their food. However, they have very large kidneys, so (unlike us) are able to produce urine which is saltier than sea-water -- therefore they can safely drink sea-water. If we tried that, we'd have to produce more urine than the water we drank, and would soon die of dehydration.
Most sea mammals are similar, though there are some sea-living ones which do need fresh water. For example, the European otter lives in rivers, but many individuals also live on the foreshore. These sea-living otters must return to fresh water every day to drink and bathe (they are of course not the proper sea-otter, which is a different, full-time marine mammal from the Pacific).