On April 14, 2004, about 1745 eastern daylight time, a Boeing 737-700, N448WN, operated by Southwest Airlines as flight 1025, was not damaged during an encounter with turbulence in cruise flight near Beach Haven, New Jersey. There were no injuries to the 2 certificated airline transport pilots, 2 flight attendants, and 117 passengers; while 1 flight attendant was seriously injured. Instrument meteorological conditions prevailed for the flight that departed Baltimore, Maryland; destined for Long Island MacArthur Airport (ISP), Islip, New York. An instrument flight rules flight plan was filed for the scheduled passenger flight conducted under 14 CFR Part 121. The captain reported that the airplane encountered moderate turbulence in cruise flight at 21,000 feet, near the BRIGS intersection. The passengers were seated and the fasten seatbelt sign was illuminated. The flight attendants were also instructed to take their seats. While attempting to sit down, a flight attendant fell to the floor and suffered a fractured ankle. The airplane subsequently landed uneventfully at ISP about 1805.