I did a small Hartree-Fock computation on H2O and H2O-.
For H2O: O-H distance is 0,95Å and H-O-H angle 105,5 (compare with measured distance 0,9584Å and angle 104,45)
For H2O-: O-H distance is 1,03Å and H-O-H angle 99,9
The extra electron goes into an anti-bonding orbital which push the two hydrogen atoms closer to one another. Not surprisingly, this anion is not stable.