Visible light communication based on light-emitting diodes (LEDs) has become a promising candidate by providing high data rates, low latency, and secure communication for underwater environments. In this paper, a self-designed common-anode GaN-based five-primary-color LED (RGBYC LED) on a Si substrate is proposed and fabricated. The design of a common anode is used to mitigate the saturation effect for a low-frequency component. Additionally, compared with commercially available LEDs that suffer from nonlinearity distortion, applying the designed LED can provide much better and broader linearity according to the measurement results. Therefore, the modulation depth and system performance can be further improved to implement a high-speed underwater visible light communication (UVLC) system. There is no nonlinearity compensation algorithm applied due to the good linearity of the proposed LED; thus, the offline digital signal processing is simplified. We experimentally demonstrate 14.81 Gbit/s 64 quadrature amplitude modulation (QAM)-discrete multitone (DMT) and 15.17 Gbit/s bit-loading-DMT transmissions through a 1.2-m-long underwater channel based on the proposed RGBYC LED with an intrasymbol frequency-domain averaging channel estimation and zero-forcing equalization. As far as we know, this is the highest data rate for an LED-based UVLC system.