Many robots designed to operate on or in water are destined to serve in military, naval, and homeland security capacities. Others are aimed at scientific exploration and data gathering, such as monitoring marine ecosystems and gathering data about water quality. Some of the plainer ones are remotely operated vehicles (ROVs), underwater unmanned vehicles (UUVs), or autonomous underwater vehicles (AUVs): the naval versions of their unmanned ground vehicle (UGV) and unmanned aerial vehicle (UAV) counterparts. Others look like fish, jellyfish, or submarines.
Whether they navigate the salty seas or fresh water lakes, rivers, or oceans, many models can do a number of different types of tasks, depending on their payloads. Robots made to work in water are usually designed to be either remote-controlled or autonomous, and some can even switch from one mode to another.
Click on the image below to see 12 of these underwater workers.
The Serpent remotely operated vehicle (ROV) from Seaview Systems is designed for exploring very small-diameter pipelines. It can investigate conduits as small as 9 inches (23 cm) in diameter, and fit around bends with a radius as narrow as 27 inches (68.5 cm). Measuring 9 inch x 9 inch x 57 inch (23 cm x 23 cm x 145 cm) and weighing 70 lb (32 kg), the Serpent runs on two 300W brushless DC motors that give it a total forward thrust of 18 lb (8 kg). With a 0.5 inch (1.3 cm) diameter fiber-optic tether, it can explore as far as 6,000 ft (1,830 m) down a pipe or tunnel. A 360-degree pan/orbit/zoom color camera and two color cameras are included, along with two 70W high-intensity LEDs. The robot also has heading, pitch and roll, and depth sensors, as well as sonar. A fiber-optic telemetry system provides up to three video channels, four RS232 channels, and two RS485 channels. (Source: Seaview Systems)
Rob, shape matters a whole lot in the water and under it, much more than it matters in air. Not only that the greater density takes a lot more power to move it out of the way, but also that the friction of moving through water is greater. One large difference though is that water is generally not compressible,at least not like air.
William is right about shape mattering even more in water than it does in air as far as a fish--or a robot's--speed, maneuverability, and efficiency and therefore power consumption. Just think how much harder it is to swim through water than to walk through air, and the muscles swimming gives your arms as a result.
Rob, sometimes efficiency and streamlining are not the only consideration. Watch some of those fish at the aquarium, some very big fish disappear when they are viewed from the front or the rear. And you don't see very many fish with missile launching abilities.
Ann, watch some of those fish in the big aquarium at your local zoo. There are a few of them that are huge when viewed from the side, and they really do become hard to see when they turn and swim away. My guess is that it is that way to confuse predators.