Hybrid aerial-aquatic vehicles have the potential to transform marine ecosystem monitoring, offshore infrastructure inspection, environmental surveying, and search-and-rescue operations by seamlessly transitioning between air and water. However, a persistent challenge limiting submerged operation is maintaining reliable communication: radio-frequency (RF) signals are rapidly attenuated underwater, and acoustic communication su0ers from limited bandwidth and high latency. As a result, a tether (a physical cable linking the vehicle to the surface) remains essential for real-time command and data transmission. This work presents the design and implementation of a novel dynamic tether management system that enables continuous remote control of an aerial-aquatic vehicle through a surface buoy. Mounted directly on the vehicle, the system actively regulates tether length using a custom tension-sensing feedback mechanism to keep the buoy at the water surface while preventing excess slack that could entangle the vehicle. The tether system is integrated into a custom-built quadcopter-based platform featuring waterproofed electronics and tiltable thruster arms for underwater thrust vectoring. The complete system was validated in three operational underwater modes: descent, ascent, and horizontal translation. This demonstrated stable communication and tether control throughout. By addressing a critical limitation in underwater remote control, this work advances the design of hybrid platforms capable of precise operation across aerial and aquatic environments.