1. An underwater detection device for detecting underwater by transmission and reception of an ultrasonic signal, comprising: a transmission module for transmitting the ultrasonic signal underwater; a reception module for receiving an echo signal of the transmitted ultrasonic signal; a bottom detection module for detecting a bottom based on the echo signal from the bottom; a bottom-sediment determination module for calculating probabilities of the bottom sediment on how much the bottom sediment contains each of predetermined bottom sediment types based on the echo signal; and a bottom-sediment display module for displaying a texture representing each of the bottom sediment types below the bottom being displayed based on the probabilities.