The Fisher King is a servant of the Lady of the Lake, who lives in a modest hut on the lakeshore outside Murky Waters. He also seems to be the postmaster for the village and handles the delivery of a letter from Geralt to his lover to Vizima.
He can usually be found in or near his hut. During the day, he either fishes (surprise!) or sits on his stool outside his hut contemplating whatever it is a Fisher King might contemplate. It is here that Geralt finds him to ask about borrowing his boat to get to Black Tern Island. His silent nature could make this a problem, but thankfully does not.
He is not a very talkative guy, and manages to communicate mostly with a series of Mhm's and Uhm's. Sometimes it is more like twenty questions than a conversation, but when push comes to shove, he can speak, he simply chooses not to, mostly.
There is some suggestion that he is more than just a servant of the Lady of the Lake. The hermit says that the Fisher King and the Lady of the Lake take care of Murky Waters. Additionally, the lady herself suggests that he was her lover, on at least one occasion. On this topic, if Geralt earns the Lady's most intimate reward, he can afterwards ask the question "Why the Fisher King?" which leads her to playfully belittle Geralt's sexual prowess by comparison.
Blood and Wine expansion
Where the Lady is, there are her faithful servants and thus even Fisher King made his way to the duchy of Toussaint. His ultimate task is unknown, but there is his faithful boat near him so he is likely transporting common but worthy folks on Lac Célavy to Lady's islet.
In terms of eloquence nothing changed at all, he is still that calm old man who is fishing and sometimes responds with Mhm's or Hm's. He can also be sometimes seen collecting herbs.