The Hawks scored with less than five minutes to play in the first half.
Watertown had trouble getting anything going as four Purple Tigers were red-carded, forcing the home team to play four men shorthanded.
“It’s just heartbreaking,” Watertown coach Jestin Gutierrez said after the Tigers’ fourth season ended 10-4-1. “You can’t play, you can’t be aggressive when you only have seven going against 11.
“You just have to come back and get better next year, get mentally stronger.”