As forecasters warned on Wednesday of near-record water levels swamping North and South Carolina, millions of people were under evacuation orders while Hurricane Dorian crept up the south-eastern US coast.

Dorian’s ferocity has weakened since it struck the Bahamas. But it is still powerful and appears likely to get dangerously near Charleston, South Carolina, which is particularly vulnerable since it is located on a peninsula. Businesses are boarded up around the city and some people have been in shelters for days.

Governor Henry McMaster warned residents in evacuation zones to “get out now”. There was still time for people to leave at-risk areas, he said, but they should do so immediately.

In the Bahamas the storm brought chaos and terror during the day-and-a-half mauling of the northern islands, leaving behind a muddy, debris-strewn landscape of smashed and flooded-out homes on Abaco and Grand Bahama. Senior government officials told the Guardian that the official death toll remains at seven but it is expected to rise. Several outlets reported 20 dead on Wednesday.

On Monday, McMaster ordered 830,000 to leave areas likely to be affected. Charleston was among the mandatory evacuation zones, along with parts of counties to the north.

“Once the wind speeds reach up to 40mph, we can no longer come in to get you,” McMaster said, according to the South Carolina news channel WBTW News 13. “It is the water that kills people. It is the water that is the real danger. And it is clear we’re going to have a lot of water.”

Chris Creel, manager of Piggly Wiggly, stocks pallets of bottled water as grocery customers prepare for the arrival of storm weather with Hurricane Dorian in New Bern, North Carolina. Photograph: Gray Whitley/AP

A flood chart by the National Weather Service projected a combined high tide and storm surge around Charleston Harbor of 10.3ft. The record is 12.5ft, set by Hurricane Hugo in 1989.

Dorian, now a category 2 storm, was still off the eastern coast of Florida on Wednesday, moving slowly to the north. A hurricane warning covered about 500 miles of coastline.

Approximately 396,000 North Carolina residents were under mandatory evacuation orders, according to a state joint information center spokeswoman.﻿

An aerial photograph released by the UK Ministry of Defence (MOD) on Wednesday shows destruction in the aftermath of Hurricane Dorian on the island of Great Abaco in the northern Bahamas on 3 September. Photograph: Lphoto Paul Halliwell/AFP/Getty Images

Many were in shock as they slowly came out of shelters and checked on their homes. In one community, George Bolter stood in bright sunshine and surveyed what was once his home. He picked at the debris, trying to find something salvageable. A couple of walls were the only things left.

“I have lost everything,” he said. “I have lost all my baby’s clothes, my son’s clothes. We have nowhere to stay, nowhere to live. Everything is gone.”

The Bahamian government sent hundreds of police officers and marines into the stricken islands, along with doctors, nurses and other healthcare workers, in an effort to reach drenched and stunned victims and take the full measure of the disaster.

“Right now there are just a lot of unknowns,” the parliament member Iram Lewis said. “We need help.”

The US coast guard, Britain’s Royal Navy and relief organisations including the United Nations and the Red Cross joined a burgeoning effort to rush food and medicine to survivors and lift the most desperate to safety. The US government dispatched urban search-and-rescue teams.