What are the main types of underwater robots, and what are their respective characteristics?

Okay, talking about underwater robots, it's actually not that complicated. You can think of them as two main categories, plus a few "branches."

This is the most common type. You can imagine it as an "underwater kite" or a robot dog on a leash.

Characteristics:
- Has a long tether: This cable is its lifeline, connecting it to the operating vessel on the surface. The operator on the ship controls it like playing a video game, watching a screen and moving joysticks.
- No power worries: Electricity and signals are continuously supplied through the tether, so theoretically, as long as the ship stays put, it can work indefinitely.
- Can perform delicate tasks: Because it's remotely controlled in real-time by a human, it can carry out very complex operations. For example, equipped with robotic arms, it can tighten screws, cut cables, or retrieve objects (like airplane black boxes) underwater. Many deep-sea archaeological missions, shipwreck salvages, and underwater equipment inspections are handled by ROVs.
Disadvantages:
- Limited range: Its operational area is essentially restricted by the length of the tether, like being leashed. Also, the tether can easily get snagged on underwater objects or be affected by currents.

This one is cooler, like an "oceanic robot vacuum cleaner" or a "smart torpedo."

Characteristics:
- Completely wireless, free to roam: Before deployment, it's programmed with a mission and route, for example, "go to this sea area, sweep back and forth, and map the seabed." Then it's launched, and it executes the plan autonomously, returning after completing its work.
- Suitable for large-scale reconnaissance: Because it's not constrained by a tether, it can travel very far, making it ideal for mapping large areas of the seabed, exploring underwater resources, or conducting military reconnaissance.
Disadvantages:
- Generally no real-time interaction: Once launched, it's in an "offline" state; you can't command it in real-time. It relies on its own intelligence to handle unexpected situations, and if something goes wrong, it might not return.
- Limited battery life: It operates on its own batteries, so the duration of a mission is limited.
- Cannot perform delicate tasks: Its primary function is to "see" and "listen" (using sonar and other sensors), not to stop in one place and manipulate objects with robotic arms like an ROV.

Underwater Glider: This is a particularly power-efficient type of AUV. It has no propeller; instead, it moves like a fish by adjusting its buoyancy (absorbing or expelling water) to sink and rise, and then uses wings to convert this vertical motion into forward propulsion. It moves slowly but is extremely power-efficient, capable of drifting in the ocean for months and covering thousands of kilometers, specifically for collecting long-term oceanographic data (water temperature, salinity, etc.).
Hybrid (HROV): Combines the advantages of both ROVs and AUVs. It can operate autonomously like an AUV, but when delicate operations are required, a thin fiber optic cable can be deployed from its stern to the mother ship, temporarily switching it to ROV mode. Of course, this type is more complex and expensive.

In summary, you can remember it this way:

For delicate, complex tasks, or when you need to stay in one place for extended operations, use a tethered ROV.
For large-scale, long-distance mapping or exploration, use an untethered AUV.