What percentage of the water in the human body is inside the cells?

The human body is made up entirely of cells. There is no "outside" cells, so the answer is technically 100%. There is nothing other than cells in the human body.

From http://www.faqs.org/nutrition/Smi-Z/Water.html
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The amount of water in a human body depends on age, gender, body type, and level of physical activity. The bodies of infants up to about twelve months of age contain about 58 percent water; the bodies of children six to seven years of age are 62 percent water; teenage boys are about 59 percent water; and teenage girls are about 45 percent water. The body of an adult male is approximately 62 percent water, while an adult female is 51 percent water. Physically active individuals generally have more water in their bodies than those who are less physically active. Because they sweat more, active people need to replenish water more often, thus raising their water level. A trained male runner may have up to 71 percent water in his body, while a female gymnast may have 70 percent. Obese individuals, on the other hand, have a lower percentage of water in their bodies (about 48%). Morbidly obese individuals are only about 36 percent water. In addition, the older one gets, the less water is retained in one's cells. As a result, old skin looks drier and wrinkles appear.