The 22-year-old Dutchman, who has been on loan at Leicester, was due to join the Hornets last January but turned the move down.
"I know I have to win the fans over, I'm just thankful I've been given the opportunity to come here again and that everyone still wanted me.
"I'm looking forward to doing what I do, scoring goals.
"That's what I want personally, but of course for the team automatic promotion is vital. Then when we go up, it's all about staying up this time."