The expansion of passive acoustic monitoring has generated large volumes of acoustic data in bioacoustic and ecoacoustic studies, making a sound understanding of acoustic metrics, indices, and their interpretative limitations increasingly important. This article provides a scientific and educational review of the main acoustic quantities and indices that can be extracted from sound recordings, with particular emphasis on their application in monitoring aquatic environments. First, the fundamentals of digital acoustic data and key acoustic quantities, including root mean square (RMS), peak amplitude, sound pressure level (SPL), power spectral density (PSD), dominant frequency, and spectral bandwidth, are introduced. Commonly used ecoacoustic indices, including ACI, ADI, AEI, BI, NDSI, and acoustic entropy indices, are then reviewed in terms of their underlying concepts, applications, and interpretative limitations. Acoustic indices can be useful for summarizing and comparing temporal and spatial variation in soundscapes; however, their values depend on frequency range, temporal scale, processing settings, and habitat characteristics, and they should not be interpreted as direct measures of biodiversity or ecological status without independent validation. Purpose-driven selection of acoustic quantities and indices, together with transparent methodological reporting, can support more systematic and reproducible analysis of acoustic data in aquatic environmental monitoring.