Absolute oscillator strengths (f-values) for rorational lines of the F(0, 0, 0)--X(0, 0, 0) band of H/sub 2/O at 111.5 nm have been measured with an uncertainty of +- 40% using quantitative photographic techniques. The f-value for the 1/sub 11/--0/sub 00/ line, the one most likely to be seen in absorption in interstellar clouds, is (3.0 +- 1.2) x 10/sup -2/. This value is combined with a measured upper limit for absorption by this line in the zeta Oph cloud to show that the column density of H/sub 2/O in the O/sub 00/ level is less than 5.3 +- 10/sup 12/ cm/sup -2/. This upper limit is larger than the predictions of gas-phase, chemical models of this cloud. We show that definite detection of H/sub 2/O in diffuse clouds at the level of this upper limit would be in conflict with models of molecular formation in shock-heated gas. The data analysis produced additional results: the absolute scale of the oscillator strengths for lines of the C(0, 0, 0)--X(0, 0, 0) band of H/sub 2/O at 124 nm is reduced by 10%; the integrated absorption coefficients for F--X and C--X bands are compared to calculated and measured values and agree withinmore » the experimental uncertainties.« less