I think the Crab-eating frog, Rana cancrivora, is the winner for salt water tolerance. It can supposedly handle 80% of sea-waters salinity for days if it is acclimatized to it. Its tadpoles can manage water saltier than the sea. Here's the study (full pdf is even available): Osmotic Regulation in the Crab-Eating Frog (Rana Cancrivora)