No GPS, and almost no usable radio at all
Every land and aerial robot on this site has had at least the option of a GPS fix to anchor its position estimate. Underwater, that option doesn't exist in any usable form. GPS relies on decoding faint microwave signals from orbiting satellites, and water absorbs radio frequencies in that band almost immediately, within centimeters at the frequencies GPS uses. A submerged vehicle simply cannot receive a satellite signal, full stop, regardless of how sensitive its receiver is or how clear the water looks. This isn't a degraded signal problem like a robot working under a bridge or inside a building, where GPS gets noisy but occasionally sneaks through. It's a hard physical wall.
The same physics that kills GPS also kills most radio-based communication, which is why underwater systems lean on acoustic modems instead, devices that encode data as sound waves traveling through water. Sound propagates underwater far better than radio does, but it comes with serious costs: acoustic links are slow, often delivering only kilobits per second compared to the megabits or gigabits available to a surface robot, and the round-trip latency can stretch to seconds because sound travels roughly a million times slower than radio. A command sent to a distant underwater vehicle and its acknowledgment can take longer to arrive than a human reaction time, which rules out anything resembling responsive real-time teleoperation at range.