A bay is a body of water bordered on 3 sides by land, and by definition the "4th side" is the ocean. Whether it is saltwater or freshwater will depend on where the water is from. If the bay is of an ocean, it will be of saltwater. If it is of a freshwater lake or river, it will be of freshwater.
Certain freshwater fish can survive saltwater conditions; examples are black mollies, mono's and scats. All of these fish aren't truly freshwater or saltwater, they are Brackish. Brackish is between freshwater and saltwater, and so these fish are highly adaptable.
No, freshwater fish must be kept in freshwater and saltwater fish must be kept in saltwater. The one exception (that I know of) in nature is salmon which migrate from where they were born (freshwater) to the ocean and then back to where they were born again to breed.