[tstactplsignore]

You're correct, the immediate ancestors of tetrapods almost certainly lived in freshwater, as known intermediate fossils like titaalik were found in fresh water stream deposits[1]. I believe all of the immediate ancestors of and early tetrapods have been found in fresh water deposits, although I can't confirm.

[1] http://tiktaalik.uchicago.edu/searching4Tik3.html