One-photon double-photoionization (DPI) is an exquisitely sensitive experimental probe of electron correlation. For more than three decades, DPI theoretical and experimental efforts have produced a rich literature investigating simple atoms and diatomics like H2, but remained stagnant in terms of complexity of the systems. No complete experiments (measuring body-frame double-ionization- differential-cross-sections) of polyatomic systems with more than two electrons were done prior to this work because the residual dications did not dissociate in their ground and low lying states required by the COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) experimental technique. A COLTRIMS experiment performed at the Advanced Light Source (ALS) found that the water di-cation does dissociate completely in its ground and low lying excited electronic states, opening the door to such studies. This successful DPI experiment on water also allows an investigation into the differences between body-frame inter-shell (different orbitals) and intra-shell (same orbital) electron correlation for the first time. For example, the 1b1 orbital is the lone-pair orbital of water that is perpendicular to the plane of the molecule and, therefore is expected to produce a DPI intra-shell cross-section that resembles an atom’s cross-section. The other lone-pair orbital of water, the 3a1 orbital, is in the plane of the molecule and, therefore is expected to produce a DPI intra-shell cross-section that has a structure more indicative of a molecule due to the correlated electrons’ motion perturbed by the the nuclei. Moreover, a DPI example of inter-shell cross-section could be one electron from the 1b1 orbital and the other correlated electron from the 1a1 orbital, which has never been calculated before for the body-frame. Thus, an ab initio investigation of DPI of water provides a path to new physics not yet explored and will further our understanding of electron correlation that is fundamental in electron dynamics in atomic and molecular physics. However, the first crucial step in this theoretical pursuit is to clearly define the molecular frame at the moment of photoabsorption to investigate multi-orbital electron correlation with respect to this defined frame of reference and the work reported in this thesis provides this necessary first step. This overarching theme of understanding electron correlation within the body-frame for more complex polyatomic and in the continuum after the two correlated electrons are ionized on an ab inito level is the new standard for DPI studies and this dissertation lays the foundation for that standard by investigating the dynamics of D2O++ breakup after double-photoionization while unambiguously defining the molecular frame. To begin, unraveling all the nuclear dynamics by classical trajectories propagated on the nine dication states of D2O++ that are energetically accessible for double ionization by a 61 eV photon, is presented. Excellent agreement between theory and experiment is obtained and a state-selective analysis of the electron correlation is provided by these theoretical simulations. Breakdown in the axial-recoil approximation for some states of D2O++ is discovered, and a blueprint is defined for guiding experimental observations that indicate this breakdown.Agreement between theory and the single-photon double-ionization experiment encouraged a comparison between intense field multiphoton double ionization of D2O and these classical trajectory simulations. The dynamics of intense-field multiphoton ionization is at the heart of modern attosecond and femtosecond molecular physics. Here it is shown how difference between the dynamics of single-photon double ionization of water and those of intense field double ionization are revealed by the final momenta of the fragments. In this work a detailed picture of the combined nuclear and electronic dynamics of intense-field double ionization on the femtosecond time scale begins to emerge. Finally, a look into propagation of classical trajectories with initial conditions sampled from a Wigner distribution built from a vibrationally excited wavefunction is presented. The vibrationally excited wavefunction will contain nodes that in-turn produce negative regions in the generated Wigner distribution. The volume of negativity in the Wigner distribution is a signature of quan- tum character and, therefore this study explores probing quantum character using a semi-classical technique. Interesting connections between classical and quantum behavior is reported and mul- tiple theoretical predictions are stated for possible future experiments on the nuclear breakup of D2O++ from a vibrationally excited ensemble of neutral heavy water.