United Airlines flight 2122 encountered severe turbulence while in cruise flight at flight level (FL) 330 enroute to Punta Cana Airport (PUJ), Dominican Republic, and a flight attendant (FA) sustained serious injuries. The flight crew reported that after reaching a cruise altitude of FL330 and before entering oceanic airspace they downloaded the latest Weather Services International (WSI) radar image, lightning data, and SkyPath ride reports. The airborne radar was in use which depicted some very small, scattered cells well below their altitude. Shortly thereafter, the captain visually acquired a cloud buildup immediately ahead and below their altitude. With no visual indication of the cloud on the weather radar they made a right turn to avoid any potential turbulence. While in the right turn, the airplane entered visual meteorological conditions and the captain visually acquired another cloud buildup directly ahead. To avoid the buildup the captain continued the right turn to no avail. The airplane entered the clouds and immediately encountered severe turbulence for about 5 seconds. A public address (PA) announcement was immediately made “be seated immediately, be seated immediately”. Following the turbulence encountered the flight crew was notified that two aft FAs had been injured. Just before the turbulence event, the number 2L and 2R FAs had moved the beverage cart to the aft galley, so the 2R FA could use the lavatory. After entering the lavatory, she was thrown into the air and landed on her left ankle and knee. The 2L FA stated that as soon as the 2R FA entered the lavatory, they encountered severe turbulence. She and the beverage cart were tossed into the air, and she impacted the floor of the airplane on her left side. She stated that the impact was like “slamming down from a 5th floor building”. Upon being notified of the injuries, the flight crew declared an emergency and diverted to Myrtle Beach, South Carolina, for medical assistance. The two injured FAs were transported to local hospitals where one was diagnosed with serious injuries. At the time of the turbulence encounter there were no active significant meteorological information (SIGMETs), convective SIGMETs, airman’s meteorological information (AIRMETs), or pilot reports (PIREPs) of any significant turbulence.