1. An underwater detection apparatus, comprising: a transmitter configured to transmit a transmission wave; a receiver configured to obtain a receive signal based on a reflection of the transmission wave; and a hardware processor programmed to at least: detect an underwater target object based at least in part on the receive signal, estimate a density index value of objects within a given area in a vicinity of the underwater target object, the density index value indicative of a quantity of the objects within the given area, and calculate a size index value of the underwater target object based at least in part on the density index value and a target object signal intensity representing a maximum intensity of the receive signal corresponding to the underwater target object, wherein, when the density index value is above a given value, the hardware processor is further programmed to at least reduce the size index value of the underwater target object.