The entire Delaware congressional delegation and the governor as well as a contingent from Maryland descended on this sun-dappled seaside resort today to protest designation of a site 120 miles out to sea for dumping sewage sludge and industrial waste.

They told Environmental Protection Agency hearing officers that the state does not want the 7 million tons of sludge now dumped annually off New York deposited instead off the mouth of Delaware Bay.

EPA is proposing a 20-square-mile ocean area as the nation's first deep-water sewage sludge dump site. It could be the recipient of sludge from huge New York and New Jersey muncipalities if the EPA decides to close a 50-year-old dumping area 12 miles off New York, now operating on a temporary permit.

It also could provide relief for Washington, D.C., which has filed preliminary permit applications to use the site for the sewage sludge produced at its Blue Plains treatment plant.

Delaware Gov. Pierre S. du Pont argued today that designation of the new site would violate the spirit of the Marine Protection Research and Sanctuaries Act, which he helped write as a congressman.

"When we enacted it over a decade ago," said du Pont, "we in Congress viewed ocean disposal of sewage sludge, aqueous industrial waste, toxic chemicals and other wastes as a temporary, short-term solution. It was not intended that the ocean become a cesspool simply because it is an inexpensive way to dispose of unwanted materials."

The site now is in use as an industrial waste dump. But of greater concern to those testifying today than occasional industrial dumping was the prospect of barges full of New York and New Jersey sludge opening their hulls at the site on a regular basis. Officials from Delaware and Maryland coastal areas expressed concern that beaches and marine life could be damaged.

Democratic Sen. Joseph R. Biden, Jr. said the plan, "following so closely upon the proposal to designate an ocean incineration site for disposing of PCBs in our offshore waters, suggests that our coastal regions have become the EPA's favorite dumping site."

Democratic Rep. Thomas R. Carper and Republican Sen. William V. Roth Jr. also spoke in opposition in the hearings at the Rehoboth Convention Center, as did local officials and spokesmen for Sen. Paul Sarbanes (D-Md.), Rep. Roy Dyson (D-Md.), and Maryland Natural Resources Secretary Torey Brown. About 200 people attended.

A general complaint was that EPA's proposal violated the spirit of federal law, which restrains ocean dumping.

But hearing officer Steven Schatzow, director of EPA's Office of Water Regulations and Standards, said the Ocean Dumping Act calls for no dumping of sewage sludge or industrial wastes "that unreasonably degrades the environment." A court interpretation, he said, requires the EPA to include the likely environmental effects of putting the sludge somewhere else in calculating whether ocean degradation is "unreasonable."

Schatzow acknowledged that the 12-mile site off New York is "heavily degraded," with very little life except for sludge worms on the bottom. But he said it is hard to determine how much degradation is from sludge dumping and how much from other pollutants coming out of New York harbor.

He and other EPA and National Oceanic and Atmospheric Administration officials maintained that the offshore site, 6,000 to 9,000 feet deep, as opposed to 90 feet at the 12-mile site, is better suited to sludge dumping. They said sludge would disperse quickly in ocean currents and would not affect the scarce and mobile marine population.