The energy consumption of the Internet of underwater things (IoUT) nodes is a capital aspect that dramatically affects the applicability of wireless optical technologies in several scenarios, such as ocean monitoring or underwater sensor networks. Simultaneous lightwave information and power transfer (SLIPT) is a cost-effective and energy-efficient solution for energy-constrained wireless systems. Nonetheless, the reported battery-charging times for underwater operations are high, which should be improved to make this technology attractive enough to be considered for actual implementations. This paper provides a new SLIPT strategy, introducing a specific optical signal transmission scheme by controlling the transmitted direct current (DC) level component. The scheme is based on a DC-On Off Keying (OOK) modulation with an adjustable signal range (SR) to improve the energy-harvesting process and battery-charging time for underwater operations. The results reveal that the system provides a signal-to-noise ratio higher than 28 dB and the bit-error rate of less than 10−10 which is below the forward error correction limit with improved charging time around 30 minutes and 15 seconds for 5 F and 9.4 mF, respectively over 20 cm, and 63 seconds for 9.4 mF in 50 cm link distance.