A satellite photo captured in the middle of the storm shows roughly half of Grand Bahama island covered in water on Monday.
Bahamian officials said they received a "tremendous" number of calls from people in flooded homes.
The extent of flooding caused by Hurricane Dorian in the Bahamas has been highlighted by a video of a major airport underwater.
While its winds had diminished to a Category 2 storm on the five-step Saffir-Simpson scale of intensity, Dorian expanded in size and picked up speed on Tuesday.
Scientists say climate change generally has been fuelling more powerful and wetter storms, and the only recorded hurricane more powerful than Dorian was Allen in 1980, with 305kmh winds, though it did not hit land at that strength.
It pounded the northern islands of Abaco and Grand Bahama with winds up to 295 kilometres per hour and torrential rain before finally moving into open waters on Tuesday (local time) on a course for the U.S. state of Florida.
Hurricane Dorian came to a catastrophic daylong halt over the northwest Bahamas, flooding the islands of Abaco and Grand Bahama with walls of water that lapped into the second floors of buildings, trapped people in attics and drowned the Grand Bahama airport under 6 feet of water.
The Bahamas' prime minister also expected more deaths and predicted that rebuilding would require "a massive, coordinated effort". The first aerial pictures have now emerged of the destruction on the Abaco Islands.More news: Eerie Video Shows Inside the Eye of Hurricane Dorian
More news: Knife-wielding man kills 8 schoolchildren in China
More news: Trump Warns China Not to Delay Trade Talks
The storm is inching northwest and potentially up the East Coast of the USA with sustained winds of 177 kilometres per hour, dropping it to Category 2 status.
The sea has engulfed the Bahamas global airport and the airport is now under 6 feet (2 meters) of water.
While the threat of a direct hit on Florida has largely evaporated, Dorian was expected to pass dangerously close to Georgia and SC, and possibly strike North Carolina, on Thursday or Friday (local time).
In the Bahamas, Red Cross spokesman Matthew Cochrane said more than 13,000 houses, or about 45% of the homes on Grand Bahama and Abaco, were believed to be severely damaged or destroyed.
Volunteer rescuers have used jet skis and boats to reach some people as choppy floodwaters reached roofs and the tops of palm trees.
The governors of Georgia, South Carolina and North Carolina have ordered evacuations of some coastal counties.
A few hours later, Georgia Gov. Brian Kemp ordered mandatory evacuations for that state's Atlantic coast, also starting at midday Monday.
(L-R) Langdon Bryant, Marcus McClain and Torrie Bryant, 5, hang plywood over a window as they protect the home just in case Hurricane Dorian hits the area on September 01, 2019 in Port Salerno, Florida.