Recently, microstructured optical fibers have become the subject of extensive research as they can be employed in many civilian and military applications. One of the recent areas of research is to enhance the normalized responsivity (NR) to acoustic pressure of the optical fiber hydrophones by replacing the conventional single mode fibers (SMFs) with hollow-core photonic bandgap fibers (HC-PBFs). However, this needs further investigation. In order to fully understand the feasibility of using HC-PBFs as acoustic pressure sensors and in underwater communication systems, it is important to study their modal properties in this environment. In this paper, the finite element solver (FES) COMSOL Multiphysics is used to study the effect of underwater acoustic pressure on the effective refractive index neff of the fundamental mode and discuss its contribution to NR. Besides, we investigate, for the first time to our knowledge, the effect of underwater acoustic pressure on the effective area Aeff and the numerical aperture (NA) of the HC-PBF.