A GaN blue laser diode (BLD) based visible-light communication link is demonstrated in seawater environment to provide 16-QAM OFDM data transmission at 14.8 Gbps over 1.7 m. Lengthening the seawater distance to 10.2 m only decreases the transmission data rate by 4 Gbps, as caused by mirror loss, air/glass/seawater interfacial reflections and absorption/scattering in seawater. To optimize the QAM-OFDM transmission, the sampling rate of the encoded data is compromised to avoid the aliasing and oversampling effects during waveform extraction procedure. The sampling rate is optimized to 3~5 times of the encoded data bandwidth for suppressing peak-to-average power ratio (PAPR). Oversampling filters out background noise but also attenuates data amplitude to degrade transmission performance. Without using the multicarrier spectrally filtered OFDM, the 16-QAM OFDM data format only promotes the transmission capacity of BLD up to 7.6-Gbps in seawater. With spectrally filtering out the side loabs of each OFDM subcarrier, the allowable modulation bandwidth is greatly improved from 1.9 to 2.7 GHz, as the inter-carrier interference induced crosstalk between subbands is relieved to improve the SNR of the carried data with a raw data rate of up to 10.8 Gbps.