The water column is usually taken as an homogeneous velocity layer in OBN-type marine seismic surveys. Heterogeneities caused by salinity and temperature variations, however, may introduce significant artifacts resulting in inaccurate interpretation of sub-sea floor geologic features, especially accounting the long time spans of seismic acquisitions that take place in ultra-deep ocean environments. Here we investigate the underlying structure of sensitivity kernels in this horizontally distributed acquisition geometry, mostly focused on the development of improved water layer velocity reconstructions from bottom node data. Two possible formulations are here considered, relating first arrivals with velocity perturbations, where a reference oceanic state and some overall shape/pattern of the velocity perturbations could be known a priori. The results are expected to provide guidelines for adapted inversion schemes, directed to more detailed water layer velocity reconstruction.