"In 1919 a professional swimmer, C.S. Leaf, negotiated the distance between Antelope Island and Saltair in 2 hours, 28 minutes and 27 seconds, and 7 years later a marathon swim was staged; the event was won by Chick Mitchell. The marathon was revived in 1930 and for 3 years was won by Orson Spencer; his record time, 2:20 was set in 1932."
Well, here is the newspaper article printed about the race. I got the clip from Julie Bernard. The article doesn't have a date, or whether it was the Deseret News or the Salt Lake Tribune. But it implies that it was after 1931. Based on the above statement by Mr. Morgan in his book, this article was from 1932.
"Veteran Paddler [Swims] To Victory in 2:20 Flat
Ray Welch Places Second; Nielsen's [Great] Finish Gains Him Junior Prize; Maurine Wilhelmsen Clocks 3:31:39 by George Wells
Orson Spencer's powerful strokes took the sting out of briny Great Salt lake again Saturday when he stroked to victory and another new record in the annual Antelope island-to-Saltair Beach six-and-one-half-mile swim.
It was the third successive victory for the Deseret gymnasium ace, whose blond hair stands out in contrast with the bronze of his brawny shoulders.
Spencer literally cruised to the finish in 2 hours 20 minutes, to lower by 5 minutes 41 seconds the record he established in the event last year.
The veteran finished 15 minutes 35 seconds earlier than his nearest rival, Ray Welch, junior champion in 1930, who clocked 2:35:35.
Theron Nielsen, after a rousing spurt in the final stretch, won third place and the junior championship. He clocked 2 hours 44 minutes 20 seconds.
Conditions as fair as those of the 1931 race prevailed Saturday. A slight swell that lapped the swimmers gently toward their left as they proceeded in an almost due southerly direction prevailed throughout most of the race.
And, with his powerful strokes as a rudder, Spencer kept almost a bee line for the launch that designated the finish line. He went into the lead from the start, and his steady free style propelled him out in front more than 200 yards by the time the half way mark was reached.
At that point, Welch passed Don Larsen and went into second place. Welch held his position with little change in regard to Spencer, but he steadily pulled ahead of Larsen until he had left approximately 150 yards between him and his follower.
From that point on the genuine thrills of the race developed as Larsen and Theron Nielsen began a stroke for stroke battle that lasted throughout the 150-yard stretch to the finish."