A search-and-rescue crew found the life ring in waters to the northeast of Crooked Island in the Bahamas, about 75 miles (120 km) from the ship's last known position before it went missing on Thursday morning, the Coast Guard said.
A spokesman said crews will resume the search in the same area at sunrise.
"Because we found a life ring, the assumption can be made that we are searching in the right area," said Coast Guard spokesman Petty Officer Jon-Paul Rios.
"When we commence searching tomorrow morning at sunrise, hopefully we'll be able to find something else. Every little bit helps," he added.
At 8 pm ET (0000 GMT), Joaquin, which strengthened significantly early Saturday, had maximum sustained winds of 145 miles (233 km) per hour, the US. National Hurricane Center said.
The storm, a potentially catastrophic Category 4 on a scale of 1 to 5, was about 450 miles (724 km) southwest of Bermuda on Saturday night, Dennis Feltgen, spokesman for the NHC, said.
El Faro, a 735-foot (224-m) cargo ship with 28 US. citizens and five Polish nationals aboard, was headed to San Juan, Puerto Rico, from Jacksonville, Florida when it reported losing propulsion and that it was listing and taking on water, the Coast Guard said.
There had been no further communications after a distress call received at about 7:30 am (1130 GMT) Thursday, the Coast Guard said. Search and rescue efforts have covered more than 30,000 square miles since Thursday.
"We are very surprised that we lost all communication with the ship," Mike Hanson, a spokesman for El Faro's owner, Tote Maritime Puerto Rico, told Reuters on Saturday.
"The ship was simply in the wrong place at the wrong time," he added, saying Joaquin was just a tropical storm when El Faro set out from Jacksonville but later intensified rapidly into a major hurricane.
Meanwhile, vast swaths of US. Southeast and mid-Atlantic states were grappling with heavy rains and flooding from a separate weather system which has already caused at least five deaths, washed out roads and prompted evacuations and flash flood warnings. President Barack Obama declared a state of emergency in South Carolina, making federal emergency funds available.
South Carolina emergency officials said inclement weather conditions were to blame for three traffic fatalities across the state on Friday and Saturday, as flash flood warnings were issued for numerous counties. They said scores of homes had already been evacuated, including in the coastal county where the tourist destination Myrtle Beach is located.
Hurricane Joaquin swirled away from the Bahamas early Saturday, after slamming parts of the archipelago for more than two days. The storm was expected to pass west of Bermuda, well off the US. coastline, on Sunday, before heading on a north-northeast track taking it further out to sea.
Any slight eastward deviation in the forecast track could put Joaquin dangerously close to Bermuda, the NHC warned.
While Joaquin has continued to shift away from the US. East Coast, dangerous flooding triggered by heavy rainfall was expected across much of the Carolinas and parts of Georgia, Virginia and New Jersey this weekend, US. forecasters said.
It has been raining across much of the region all week, and the accumulated rainfall, coupled with more on the way from a weather system loosely connected with Joaquin, has prompted repeated flood warnings from the National Weather Service.
More than 15 inches (38 cm) of rain has fallen over the popular beach area since Friday, with more expected, the National Weather Service in Wilmington, North Carolina, reported.
"These kind of prolific rainfalls are not unprecedented, but this is definitely one for the history books," said NWS forecaster Dave Loewenthal in Wilmington.
"We have had numerous reports of road closures. We have had roads washed out, sinkholes forming," he said. "It's really a mess and we are going to have significantly more problems with multiple rivers reaching moderate flood (level) or higher."
A statement from the North Carolina governor's office said up to 500 residents of Brunswick County had been evacuated from their homes Friday night into early Saturday morning due to flooding from heavy rains and a levee failure in South Carolina.
"It's definitely a life-threatening situation," said NWS meteorologist Steve Pfaff.
"There were people that were stuck in vehicles that were flooded and water in some of the homes was up over the electrical outlets," he said.
North Carolina Governor Pat McCrory said the effect of the heavy rains on agriculture was a major concern. "North Carolina farmers have been harvesting crops at a feverish pace to minimize economic loss," the statement from his office said.
No deaths or serious injuries were reported in the Bahamas due to Joaquin, which destroyed houses, uprooted trees and unleashed heavy flooding on several smaller islands, but two deaths in the Carolinas on Thursday were linked to rain there.
It was not clear whether the deaths of four people in a small plane crash Friday near Lake Hartwell, South Carolina, was weather related.
Before an earlier shift in Joaquin's trajectory, New York and New Jersey, where Superstorm Sandy killed more than 120 people and caused $70 billion of property damage in October 2012, faced potential threats from the storm.