The head of the Boston firefighters union is under fire after he hooked up a department-owned hose to a hydrant outside his Hyde Park home to fill his neighbor’s pool, the Herald has learned. “I was just trying to help out a neighbor that was putting a pool up,” Richard Paris, president of the Boston Firefighters Union Local 718, told the Herald yesterday. “Now that I look at it, it was wrong. I messed up and I apologize.”