As for the robots, I have no idea how many 4,000m capable WROVs are operating around the world, but it is measured in the high hundreds, if not low thousands. My past and current employer alone have delivered handling equipment for some 250 ROVs combined.
(These are heavy duty unmanned underwater vehicles, tethered to a ship via an umbilical cable. Operator sits topside, sees what is going on via cameras, operating a pair of manipulator arms and assorted tools via fancy joysticks. Think a drone on steroids, Mad Max style.)
And yes, they are expensive, as is the vessel you need to support them.
(These are heavy duty unmanned underwater vehicles, tethered to a ship via an umbilical cable. Operator sits topside, sees what is going on via cameras, operating a pair of manipulator arms and assorted tools via fancy joysticks. Think a drone on steroids, Mad Max style.)
And yes, they are expensive, as is the vessel you need to support them.