| | Watertown, SD | Santa Monica, CA | United States |
| Population | 22,577 | 92,828 | 329,725,481 |
| Female Population | 49.7% | 50.6% | 50.5% |
| Male Population | 50.3% | 49.4% | 49.5% |
| Median Age | 38.2 | 41.4 | 38.4 |
| space3 | | | |
| Population - 2010 | 21,520 | 89,788 | 309,327,143 |
| Population - 2000 | 20,237 | 84,084 | 285,036,114 |
| Population - 1990 | 17,592 | 86,905 | 251,960,433 |
| Pop. 2000 to Now | 10.2% | 7.1% | 15.6% |
| Pop. 1990 to Now | 26.8% | 3.6% | 30.8% |
| space4 | | | |
| Population Density | 1,252.3 | 11,031.3 | 93.4 |
| Land Area | 18.0 | 8.4 | 3,531,905.4 |
| Water Area | 8.0 | 7.6 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Watertown, SD | Santa Monica, CA | United States |
| Age 0 to 4 | 5.9% | 4.9% | 5.9% |
| Age 5 to 9 | 6.9% | 3.7% | 6.1% |
| Age 10 to 14 | 6.7% | 3.7% | 6.6% |
| Age 15 to 17 | 3.9% | 2.6% | 3.9% |
| Age 18 to 20 | 4.2% | 1.9% | 4.0% |
| Age 21 to 24 | 4.8% | 3.8% | 5.2% |
| Age 25 to 34 | 13.6% | 19.1% | 13.8% |
| Age 35 to 44 | 12.7% | 15.2% | 12.9% |
| Age 45 to 54 | 10.8% | 13.5% | 12.6% |
| Age 55 to 59 | 6.4% | 6.9% | 6.7% |
| Age 60 to 64 | 6.2% | 6.4% | 6.3% |
| Age 65 to 74 | 9.5% | 10.0% | 9.6% |
| Age 75 to 84 | 5.3% | 5.3% | 4.5% |
| Age 85 and over | 3.0% | 3.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Watertown, SD | Santa Monica, CA | United States |
| White | 89.7% | 62.7% | 59.4% |
| Black | 1.1% | 4.4% | 12.2% |
| Asian | 0.7% | 9.1% | 5.6% |
| Native American | 2.5% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.5% | 0.9% | 0.4% |
| Two or More Races | 1.9% | 6.1% | 3.2% |
| Hispanic | 3.6% | 16.6% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Watertown, SD | Santa Monica, CA | United States |
| Married Population | 51.2% | 40.4% | 49.8% |
| Currently Married | 49.7% | 39.2% | 48.0% |
| Married but Separated | 1.5% | 1.2% | 1.8% |
| Single Population | 48.8% | 59.6% | 50.2% |
| Never Married | 30.7% | 43.0% | 33.8% |
| Divorced | 10.7% | 12.1% | 10.8% |
| Widowed | 7.4% | 4.5% | 5.6% |
| space5 | | | |
| Household Size | 2.26 | 1.96 | 2.60 |
| Households | 9,725 | 46,664 | 124,010,992 |
| Family Households | 5,562 | 19,382 | 80,755,759 |
| Married couple, w/children | 34.2% | 28.1% | 30.5% |
| Married couple, no children | 43.4% | 47.5% | 42.9% |
| Non Family Households | 4,163 | 27,282 | 43,255,233 |
| Single householder, w/children | 12.4% | 11.9% | 15.8% |
| Single householder, no children | 10.0% | 12.6% | 10.8% |