NEW ORLEANS — Relatives flew over Gulf of Mexico waters Wednesday where 11 oil rig workers died a year ago, residents gathered in prayer vigils onshore and President Barack Obama vowed to hold BP and others accountable for “the painful losses that they’ve caused.” Even as somber remembrances marked the first anniversary of the worst offshore oil spill in American history, there were reminders that lengthy legal battles lay ahead. BP filed a lawsuit alleging negligence by the maker of the device that failed to stop the spill, while the manufacturer of the blowout preventer and rig owner filed their own claims. The disaster began on the night of April 20, 2010, when the Deepwater Horizon rig burst into flames and killed the 11 men. The rest of the crew evacuated, but two days later the rig toppled into the Gulf and sank to the sea floor. Over the next 85 days, 206 million gallons of oil — 19 times more than the Exxon Valdez spilled — spewed from the well. Parents, siblings and wives of the workers — whose bodies were never recovered — boarded a helicopter Wednesday to see the waters where their loved ones perished. The helicopter took them from New Orleans out to the well site, circled around so that people on both sides of the aircraft could see and then returned to shore, said Arleen Weise, whose son, Adam, was killed on the rig. The only indication they were at the site was an announcement from the pilot, she said. “It was just a little emotional, seeing where they were,” Weise said by phone from Houston, where rig owner Transocean planned an evening memorial service.