Visible light communication (VLC) has been introduced as a complementary technology to acoustic communications for underwater applications. Underwater VLC can achieve much higher data rates sufficiently high for real-time image and video transmission. Such high data rates over underwater channels with frequency-selectivity necessitate the use of efficient multi-carrier techniques such as orthogonal frequency division multiplexing. In this paper, we consider an adaptive DC-biased optical OFDM (DCO-OFDM) underwater VLC system. The design of adaptive algorithm is formulated to maximize the throughput under error rate performance constraints. The receiver first calculates the signal-to-noise ratio (SNR) per each subcarrier. Then, based on SNR information, it determines which subcarrier should be loaded first and selects the maximum constellation size for each subcarrier while satisfying a predefined targeted bit error rate (BER). Our simulation results demonstrated that significant improvements in throughput can be obtained through link adaptation.