We begin with the term "ROV" which means remote operated vehicle. These are underwater diving and exploration vehicles decide for observation, scientific research, recovery, underwater thermal/laser survey, photography and much more. In the early beginnings of these vehicle they were exclusive to military naval forces from a few Allied forces. As the technology has evolved along with the various ways these items could be utilized. The ROV has become a staple in the privatization of underwater sea exploration, underwater drilling, infrastructure stabilization and the list continues.
Today, the skills in developing these marvel machines have now been easily accessible in the past ten years. Engineering colleges and private offshore schools have developed courses in the construction, control and the technical operation of the ROV's. These skills have made their way to the hobbyist in what are referred to as homemade or DIY (Do It Yourself) ROV's. This is due in part to more recent development in camera and communication technology; which, has greatly improved the radio control and frequency data link for use in these underwater devices.
Currently their are three types of ROV's being used in the military and private industry. They are Autonomous Surface Vehicles (above water), Autonomous Underwater Vehicles and Tethered Remote Operated Vehicles. They are developed in various sizes and ranges for certain functions and depth for the vehicles from industrial large to miniature sized, as they are categorized in their functionality and multiple motor enclosure.
Hobbyist typically create their own mini sized tethered ROV, stemming from RC motors from hobby radio control shops or stores. Some add transmitters and receivers to the design for various functions which include lights, motor rotation, steering and even mechanical claw maneuvers. Below is a video of my hobby ROV vehicle I created called the Spyder II with a video capturing my underwater lake exploration.