Probabilistic constellation shaping (PCS) is utilized to approach the channel capacity limit in discrete multitone (DMT) transmission for underwater optical wireless communication (UOWC) system. A fixed quadrature amplitude modulation (QAM) format with various probabilistic distributions is individually allocated for different subcarriers to obtain achievable maximum channel capacity in accordance with the pre-estimated signal-to-noise ratio. By using a 450-nm directly modulated laser diode (LD) with an available modulation bandwidth of ∼2.75 GHz, DMT with PCS technique is experimentally realized with a net data rate of 18.09 Gbit/s over 5 m, 17.21 Gbit/s over 25 m, and 12.62 Gbit/s over 35 m underwater transmission, giving substantial capacity improvement of 32.22%, 30.03%, and 27.55%, respectively, in comparison with the widely used regular QAM formats in DMT with bit-power loading scheme. The figure of merit of the UOWC system in terms of entropy, generalized mutual information (GMI), and normalized GMI are also presented. To the best of our knowledge, this is the first time to employ PCS-QAM-DMT in a UOWC system, and it is also the highest data rate ever reported for a single LD in UOWC.