Underwater robots face conditions unlike anything on land. Radio signals barely travel through water, so the usual wireless communication and positioning do not work, and robots rely on sound or a physical tether instead. Pressure rises sharply with depth, demanding sturdy, sealed construction.
These constraints shape everything about their design and how they are controlled, often with more onboard autonomy because constant remote control is impractical. Despite the difficulty, they reach places too deep, dark or dangerous for people, which is precisely why the engineering effort is worthwhile.