National Oceanic and Atmospheric Administration

Many U.S. homeowners are inadequately insured for natural-catastrophe damage ahead of the height of the Atlantic hurricane season.

Three major landfalling hurricanes in 2017—Harvey, Irma and Maria—revealed a widespread lack of full insurance coverage for homeowners in Texas, Florida, Puerto Rico and elsewhere. Those storms served as a wake-up call to some, but the underinsurance of Americans persists, according to regulators, trade groups and government data.

While most people have home insurance, many lack flood insurance, which is typically purchased from the U.S. government as a separate policy. Many homeowners also have home-insurance policy limits that are too low to cover the full cost of repairing or rebuilding their properties.

“Many people thought that they had a fully insured home or fully insured business” before last year’s catastrophes, said Iraelia Pernas, executive director of Acodese, an industry group for insurers in Puerto Rico. “They discovered that there were some exclusions in their policies.”

Javier Rivera Ríos, the territory’s insurance commissioner, said that a lot of the houses in Puerto Rico that suffered losses weren’t insured at all, let alone with flood coverage.

Jeremy Gundling of Friendswood, Texas, didn’t have flood insurance when his home filled with two feet of water during Harvey. His home insurance covered some wind damage to the house, where he lives with his wife and four children, but Mr. Gundling took out a loan to pay for most repairs.

An American flag flies above a pile of flood damaged debris stripped from inside a house that was flooded by Hurricane Harvey in Spring, Texas, U.S., on Wednesday, Sept. 6, 2017.

When Mr. Gundling decided to forgo flood insurance, he felt it was the right call based on recommendations from his real-estate and insurance agents.

“It was disheartening…to find out that I was severely underinsured,” he said.

After the storm, he bought insurance from the National Flood Insurance Program and expanded his home-insurance coverage. The federal policies pay up to $250,000 for damage to a residence and $100,000 for contents.

The number of federal flood policies covering Florida homes and small businesses rose 1% between August 2017 and March 2018, to 1.8 million, according to…