The midfielder's move to the City Ground is expected to be finally confirmed in the next 24 hours, with the Nottingham Post reporting the 32-year-old has agreed terms on the move. Watson was released by the Hornets last week.
Karanka told the Nottingham Post: “He is a Premier League player, he is an experienced player and a quality player. He is a man who wants to come to join us, which is the most important thing for me.
“I do not want to bring players in, just because we have offered them more money or a longer contract.
“It is the opposite. The players who have signed here have come because they want to be at Forest and they want to be part of things here. They believe in this project and that is more important than anything else."