The first Palestinian guerrilla infiltration of the Israeli coast ever attempted in daylight was thwarted today by an Israeli patrol boat in an offshore gunfight in which three men were killed.

Israeli Navy officials said the patrol intercepted a Palestinian boat off the Mediterranean coast of northern Israel at about 5 a.m. and sank it. The officials theorized that the guerrilla landing attempt was intended to attack early-morning bathers near Achziv, between Rosh Haniqra and Nahariyya, just south of the Lebanese border.

Officials said the fiberglass speedboat powered by an 80-horsepower outboard motor, was sighted about a mile offshore by the crew of a Navy Debur class high-speed cruiser. At first the crew thought it was a sport boat with civilian passengers.

When the Navy boat approached the craft and asked for identification, the guerrillas opened fire with a rocket-propelled grenade, slightly injuring one Israeli crewman, according to the officials. The Israelis returned the fire, killing all three guerrillas aboard, officials said.

Navy officials said they recovered an M16 rifle, a grenade launcher, a pistol with silencer and records indicating the guerrillas belonged to Fatah, the main wing of the Palestine Liberation Organization.

After the clash, Israeli air patrols combed the coastline looking for other PLO craft.

Army and Navy forces in northern Israel have been on alert since the central committee of Fatah, meeting in Damascas, resolved to step up its military activities to "liquidate" Israel.

In a statement issued in Damascus today, the PLO acknowledged that three of its guerrillas had been killed in a clash in the Mediterranean, but claimed that the squad had attacked a kibbutz and inflected heavy casualties.

A spokesman for the guerrilla military command said a commando unit attacked the town of Achziv, after evading Israeli coastals patrols, Reuter reported from Damascus.

(The spokesman said a fierce battle took place around Achziv -- a tourist town known to Palestinians by the name of Al-Zeeb -- and lasted more than 2 1/2 hours. The guerrilla command said it was in radio contact with the raiders throughout the operation, but did not give the size of the attacking force or the number of Israeli casualties.)

The last attempted seaborne guerrilla attack, also intercepted by the Israeli Navy, occurred Nov. 19, 1979. There were previous infiltration attempts by sea in April, June and August of 1979.