In 1900 William was living with his father and step-mother in Marshall, Marshall, IA.
Parkest W. Watson, b. OH May 1848, day laborer, marr 4 yrs
Catharine E., 2nd wife, b. IA June 1865.
John T., b. IA Feb 1896
Esther R., b. IA, July 1899
Claud J., b. IA Dec 1876, day laborer
William P., b. IA Dec 1878, painter
In 1910 William and family were living in Dayton, Sheridan, WY.
William Watson, b. IA, 36, farmer, marr 10 yrs
Irene O., wife, b. IA, 28, marr 10 yrs
Homer, b. IA, 9.
Their neighbors were Hugh, Flora and Hugh Watson.
In 1920 William and family was living in Dayton.
William Watson, b. IA, 43, farmer own farm.
Irene O., wife, b. IA, 38.
Homer, b. IA, 19.
In 1930 the family was living in Sheridan Co., WY.
William Watson b. IA, 55, small farm farmer.
Irene O., b. IA, 48.
Homer, b. IA, 29, small farm farmer.
In 1940 William and Irene were living in Chicago, Cook IL. They were living there in 1935 too.
William P. Watson, b. IA, 61, 8th Grade education, no occupation listed.
Irene, b. KS, 51, High School 2 yrs, partner manager rooming house.
William P. Watson
Born: 6 Dec 1877
Military Service Start Date: 2 May 1898, Private U. S. Army
Died: 20 Oct 1952
Interment: 24 Oct 1952, Ft. Rosecrans National Cemetery, PO Box 6237, San Diego 92166; Section P Site 2567.
Source: U. S. Veterans Gravesites ca. 1775-2006
It doesn't say where he died and I didn't find death info other than this.
The San Bernadino Public Library will search for an obit for a fee. Successful Searching!