  Observing changes in molecular structure requires atomic-scale {\AA}ngstrom
and femtosecond spatio-temporal resolution. We use the Fourier transform (FT)
variant of laser-induced electron diffraction (LIED), FT-LIED, to directly
retrieve the molecular structure of ${\rm H_2O^+}$ with picometre and
femtosecond resolution without a priori knowledge of the molecular structure
nor the use of retrieval algorithms or ab initio calculations. We identify a
symmetrically stretched ${\rm H_2O^+}$ field-dressed structure that is most
likely in the ground electronic state. We subsequently study the nuclear
response of an isolated water molecule to an external laser field at four
different field strengths. We show that upon increasing the laser field
strength from 2.5 to 3.8 V/{\AA}, the O-H bond is further stretched and the
molecule slightly bends. The observed ultrafast structural changes lead to an
increase in the dipole moment of water and, in turn, a stronger dipole
interaction between the nuclear framework of the molecule and the intense laser
field. Our results provide important insights into the coupling of the nuclear
framework to a laser field as the molecular geometry of ${\rm H_2O^+}$ is
altered in the presence of an external field.
