Phillip Keene, director of corporate communications for Walmart, said that during the storm, the company had safeguarded pallets of water on cargo ships that were sent out to sea and away from Maria’s path. Since then, the company has been delivering about six million bottles of water a week to Puerto Rico from the continental United States, and it is making plans to double the supply as soon as possible. Mr. Keene added that before the storm, Walmart stores in Puerto Rico generally sold about 300,000 cases of water a week, almost all of it bottled from sources on the island.

“It’s pretty amazing,” he said. “There’s a real sense of urgency.”

Some Puerto Ricans, particularly those in rural areas, are relying entirely on water provided as emergency aid by the Federal Emergency Management Agency. The water is sent to local distribution centers, and then delivered door to door by local governments.

Federal officials said that as of Wednesday, more than six million liters of water had arrived on the island, but that damaged ports, roads and bridges had slowed the deliveries, especially to the interior of the commonwealth. Officials say 75 percent of the island’s ports are open, and they are receiving about 1,100 containers of supplies a day, close to the 1,400 that came in daily before the storm.

The lack of water is far worse than anything experienced in Florida and Texas after Hurricanes Irma and Harvey. Relief experts say that because of the extent of damage to Puerto Rico’s water systems, the scale of the overall destruction and the difficulty of delivering aid to an island rather than on the mainland, it did not make sense to compare the response in Puerto Rico with Florida or Texas in terms of efficiency or focus.

“What happened in Texas and Florida were disasters,” said W. Craig Fugate, who was FEMA administrator under President Barack Obama. “What happened in Puerto Rico was a catastrophe.”

Before Maria hit, most of the bottled water consumed in Puerto Rico was produced in factories on the island, according to Manuel Reyes Alfonso, executive vice president of the Puerto Rico Chamber for the Marketing and Distribution of the Food Industry, a trade association of grocery manufacturers, distributors and store owners. Most of those facilities lost power during the storm and are now operating on generators, which allow them to produce only a small fraction of their normal output.

The island was already running low on bottled water before the storm, because it was exporting a lot to places that had been damaged by Hurricanes Harvey and Irma, Mr. Reyes Alfonso said.

The Puerto Rican government said it had taken steps to make more water accessible, like allowing the two largest dairy companies, Suiza Dairy and Tres Monjitas, to bottle potable water in milk containers.