Gov. Harry Hughes asked President Reagan today to declare the Chesapeake Bay an economic disaster area because of severe icing that has reportedly cost Maryland fishermen about $3 million worth of business this month.

The disaster declaration would commit the federal government to paying unemployment benefits to 6,000 watermen who have been unable to fish in the bay or its rivers and creeks for about three weeks.

Hughes said the Chesapeake is now 100 percent covered with ice in Maryland waters north of the bay bridges, and about 60 percent below the bridges. The rivers and creeks are 100 percent iced, as are all shellfish harvesting areas, the governor said.

State officials said the icing of the bay in the last three weeks is the worst since 1977, the last time the federal government declared a disaster area there. Hughes predicted that Reagan will grant his request, despite the governor's recent criticism of the president's social policies.

"I think this qualifies as a safety net," Hughes said, tongue-in-cheek. The governor said his $3 million estimate for financial losses came from state Department of Natural Resources officials and the state Watermen's Association.

Under a disaster declaration, fishermen thrown out of work because of the ice would become eligible for Small Business Administration loans as well as $140 a week in unemployment benefits. These benefits, normally paid in part by the state, would be funded fully by the federal government under the declaration, Hughes said.

State officials said it will take two or three weeks of above-freezing temperature for normal fishing to resume in the bay. The problem has persisted despite ice-breaking operations by state crews attempting to clear the harbors near Annapolis, Rock Hall, Kent Narrows and other areas.