ROVs: real-time control at the cost of a tether
A remotely operated vehicle, or ROV, solves the underwater communication problem by refusing to accept it: it stays physically connected to a surface ship or platform through a tether, a cable that carries power, video, and control signals in both directions. Because the tether is a wired link rather than an acoustic one, it supports the bandwidth and low latency needed for an operator to watch live video and steer the vehicle in real time, which makes ROVs the natural choice for tasks that need a human making judgment calls on the fly, like inspecting a specific structure up close, operating a manipulator arm to retrieve an object, or responding to something unexpected on camera.
The tether is also the ROV's main limitation. It restricts range and maneuverability, since the vehicle has to manage cable drag and avoid snagging it on structure, and it requires a surface ship or platform to stay on-station for the duration of the mission, which is expensive and limits how long or how far an ROV mission can practically run. Power delivered through the tether does mean ROVs aren't constrained by onboard battery life the way untethered vehicles are, so they can often run thrusters and lighting continuously for long dives, but that advantage comes bundled with the logistical cost of keeping a crewed vessel and cable-handling equipment on site the whole time.