This article is from the Dolphin
FAQ, by Jaap van der Toorn
There are a number of dolphin species that live in fresh water.
They all belong to the river dolphin families. These are: the Platanistidae (Ganges and Indus river dolphins), the Iniidae (the boto or Amazon river dolphin) and the Pontoporiidae (the baiji and the franciscana). There is one species that can be found both in fresh water (the Amazon river) and in coastal sea waters: the tucuxi (Sotalia fluviatilis).
In general, salt water species don't do well in fresh water.
They can survive for some time, but they will be come exhausted (since they have less buoyancy in fresh water) and after a while their skin will start to slough (like our own skin after spending a long time in the bathtub).
P.G.H.Evans (1987) The Natural History of Whales and Dolphins. Christoper Helm Publishers, London.