The Soviet Union could lob nuclear warheads into American coastal waters to generate giant tidal waves that would destroy U.S. submarines, Pentagon witnesses revealed yesterday.

This peck into formerly secret studies came as the Defense Department tried to convince Congress that there is no alternative to building the MX blockbuster land missile to preserve the U.S. deterrent force.

Several scientists have instead advocated building a missile submarine to cruise along the American Coast.

Current planning calls for the MX to be based on land in Nevada and Utah, but residents there are protesting the deployment plans.

Chairman K. Gunn McKay (D-Utah) of the House Appropriations military construction subcommittee called a hearing on the MX yesterday to see if there is an alternative to the racetrack deployment scheme, which would have a tremendous impact on his state.

William J. Perry, defense undersecretary for research, told the panel that the Pentagon had looked at MX alternatives and found them wanting.

If missile submarines were deployed, Perry said, studies have shown that the Soviets could generate a 40- to 50-foot high tidal wave by exploding a nuclear warhead in the ocean. The same kind of wave would be racing under the surface of the sea, Perry said, building tremendous destructive force. "It would simply turn over a submarine and destroy it," he said.

The same kind of underwater nuclear explosions could destroy U.S. missiles in such places as the Great Lakes, Perry added. Putting missiles in the Great Lakes has been suggested off and on for years.

Seymour L. Zeiberg, deputy undersecretary of defense for strategic systems, said during a break in the hearing that the Soviets could generate a 100-foot-high tidal wave with the nuclear warheads they already have. He added that the Defense Nuclear Agency had conducted exhaustive studies on the underwater explosions, studies that advocates of the coastal submarine did not know about.

Perry gave the subcommittee a glimpse into another grim scenario of nuclear warfare in putting down the idea of deploying the MX on airplanes.

The soviets could fire nuclear missiles to explode in midair, Perry said, which would "destroy every airplane in a half a million square miles."

Zeiberg said that a nuclear bomb going off above or below an airplane would send out shock waves that would rip off the plane's wings, while a blast at the same altitude would tear off the vertical tail. Perry said it would take "a battleship of the air" to make an MX airplane viable.

Defense Secretary Harold Brown, in giving the rationale for going ahead with the land-based MX deployment plan, said it would address "the most disturbing feature" of the nation's defense posture. That feature, he said, is the growing vulnerability of the current force of 1,000 Minuteman intercontinental ballistic missiles.

In contrast to the Minuteman, which stays in one underground silo, the MX would be carted from one shelter to another to keep Soviet gunners guessing about its location. The Pentagon feels that the Soviets would have little temptation to knock out 200 MX missiles in 4,600 shelters.

Brown stressed that the United States should preserve the "triad" of strategic forces -- land-based missiles, submarines and bombers. The way Soviet accuracy is improving, he added, the Minuteman force will be vulnerable "in a matter of a year or two."

Brown estimated that it would cost $34 billion, in fiscal 1980 dollars" to build the 200 MX missiles. He said this was comparable to the cost of building the Minuteman force.

Ranchers and others in Nevada and Utah fear that they will lose land if the MX is deployed in their states. And politicians complain that the invasion of 100,000 workers would overwhelm their schools, towns, and water supplies.

This facet of the problem is to be examined at a hearing today.