Three persons suffered minor injuries yesterday after an explosion and fire aboard a 40-foot cabin cruiser that had just taken on fuel at the Columbia Island Marina in the Pentagon Lagoon, D.C. police reported.

The incident occurred shortly before noon aboard the Cap'n Sam, owned by Sam Rorex, of 1801 Great Falls St., McLean, a retired Navy captain and former commander of an aircraft carrier, police said.

The cause of the explosion and fire, which sent flames leaping 25 feet in the air, was not immediately known. The blaze was extinguished by the crew of a D.C. fireboat.

Rorex's wife, Loraine; his daughter, Diana, and a guest on the boat, Alice Adamo, were treated for minor injuries at George Washington University Hospital and released.

Police said Rorex told them he had taken on 200 gallons of fuel and, as a precaution, had turned on exhaust blowers and checked for fumes. He started the port engine, which operated, and the starboard engine, which stalled. After an interval, he restarted the starboard engine and the explosion occured.

Attempts to quell the blaze with fire extinguishers proved futile and those aboard the burning boat jumped into the lagoon. Mrs. Rorex dipped below the surface, but her husband brought her to safety police said.

In the meantime, the boat had been cut loose from its moorings and was drifting toward other boats in the marina. Harbor police, who had been working nearby, arrived quickly and put a line aboard the Cap'n Sam and took her in tow. The line caught fire and broke. Another line was put aboard and police towed the boat toward the far shore of the lagoon.

At about this time, a D.C. fireboat entered the lagoon, which is within the District, through an inlet north-west of the 14th Street Bridge. The fire was extinguished, but police said the boat, valued at between $35,000 and $40,000, was a total loss.

It was towed to the harbor police station at 5th and Water Streets SW, where the cause of the incident is to be investigated, police said.