The devastation exposed deep poverty, racial tensions and federal incompetence as thousands of people were left stranded without aid. About 1,500 people were killed on the US Gulf Coast and $80 billion in damages made Katrina the costliest US natural disaster.
Katrina and Hurricane Rita, which followed it, also wrecked more than 100 oil rigs.
In New Orleans, officials paused their Gustav preparations to mark the Katrina anniversary with a symbolic burial for more than 80 victims still unidentified three years later.
Louisiana authorities warned residents to prepare to evacuate and laid on transportation for those who do not have cars. Federal officials say the levees are stronger but gaps still exist that make vulnerable some of the neighborhoods hardest hit by Katrina's floods.
Gustav barged into Haiti as a hurricane on Tuesday and killed 59 people, and eight in the neighboring Dominican Republic. It then weakened to a tropical storm and went over Jamaica but may still have killed as many as 10 there.
"I do not want to speculate, but I am fearful that we could be looking at a number in the region of 10 in terms of the number of people who have died," Jamaican Prime Minister Bruce Golding told reporters.
Floodwaters left residents stranded atop their roofs in Gordon Town near the capital, Kingston, and police enforced curfews in some north coast towns to curtail looting.
"The water is rising fast and there is widespread looting down here," resident Jackie Thompson told Reuters from Montego Bay. "The people are even stripping the material from one of the bridges. It is awful."
In the British territory of the Cayman Islands, which has not completely recovered from a near-direct hit by Hurricane Ivan in 2004, residents scurried to make last-minute purchases but only a few gas stations remained open as Gustav's rains began early on Friday afternoon.
The streets of the capital, George Town, were mostly deserted, a stark contrast to the traditional weekday bustle of the financial services industry hub.
Energy traders also watched Tropical Storm Hanna, 280 miles (450 km) north-northeast of Puerto Rico. The storm was moving west-northwest with maximum sustained winds at 50 mph (85 kph) and it could become a hurricane by next week.