In general, marine mammals hydrate from their food and don’t drink at all. There are exceptions, but for sea lions in particular, drinking is a sign of ill health, including kidney failure - see Leptospirosis | The Marine Mammal Center for some details.
If I understood correctly what a marine biologist was telling me, the two major problems salt water mammals in fresh water are susceptibility to diseases and organisms that live in fresh water, and maintaining the proper balance of salt internally. They apparently concentrate salt in urine and may do so to an excess. But the change of environment is not fatal in itself, instead it creates a stressful condition that weakens the animal, leaving susceptible to other problems.
Curiously, there are populations of manatees in Chad and Mali, both landlocked countries. The Mali ones live around 2000 kilometres up the Niger from the sea, and the Chad ones (assuming they’re still extant) live in a couple of lakes and also rivers that drain into Lake Chad. Both populations are standard West African manatees, ifaik.
There are freshwater seals in lake Saimaa in Finland.