A chaotic modulation lidar for underwater ranging is proposed and demonstrated experimentally. The 532 nm chaotic laser is generated by a chaotic electrical signal produced by a Boolean chaotic circuit directly modulating the diode laser. It is divided into two beams as the probe light and the reference light signal, respectively. The probe light signal is injected into a water tank of 1.5 m×0.3 m×0.3 m to directly illuminate the underwater target. The probe light reflected from the target and the reference light are separately converted into the electrical signals by the photoelectric detector. The distance of the underwater target can be achieved by the cross-correlation between both of the electrical signals. Through the theoretical experiment demonstration, the minimum and maximum detection distance of 0.14 m and 1.36 m are obtained respectively. The range resolution is 51 cm and the average error of the target ranging results is within 2.3 cm. The chaotic lidar can be a new technology of underwater ranging owing to its essential characteristics of the small measuring blind area and the simple structure.