Data cables, both computer to local peripheral (typically USB or serial) and between computers (typically Ethernet) have fairly strict limits on length. It is rare that a single computer will reach those lengths, but it is possible. A basic Ethernet cable can be 328 feet/100 meters long, which, given the rule of thumb that the twists and turns of cables make the required cable lengths three times the point-to-point distance, might push the length limit on a large boat.
Data cables, even of proper length, may be susceptible to electrical interference, unless they are optical fiber rather than copper. You can minimize interference, however, by following a number of rules of thumb. Sometimes, violations of the rules may need an experienced eye to spot.
The ordinary kind of hardware -- nuts and bolts -- surprisingly can act as a source of electronic interference.
Warning: engineeringspeak follows. When two electrical signals of different frequencies meet, they intermodulate, which produces at least two additional frequencies: the sum and difference of the original signal. There may also be lower-power harmonics of 2X, 3X, etc., multiples of any of these signals. The sum and difference, or harmonics of the original signals and/or sum and difference, may be generated, causing an apparently unrelated signal to leak into electronic equipment. Corroded hardware, such as a rusty bolt or guy wire, can be a place where inadvertent intermodulation happens.
When looking for non-obvious sources of electromagnetic interference, power down high-level devices for safety, and replace corroded hardware that plausibly might be inside the electrical field. If there are dissimilar metals with corrosion between them, break the possible intermodulation by putting a nonmetallic insulator between them.
Materials that can be magnetized should be not be kept within 3, and preferably 6, feet of a magnetic compass. Since not all metals can be magnetized, but many can, it's good practice just to keep large metal objects awy from the compass. This is one good reason to put the magnetic compass on the overhead, or the top of a window, to keep it away from other electronics and hidden magnets in the wheelhouse.
What sorts of magnets are hidden? Speakers and microphones both use magnets. Some power supply transformers throw off strong magnetic fields.
Before picking the mounting location for magnetic compasses, you might want to take a good-quality handheld compass, note the position of north while ashore, away from magnets, and then move it near the intended permanent compass location. If the needle deviates from what you saw as magnetic north, it's probably a bad place to mount the compass. Some compasses do have metal magnetic adjustments that let you compensate for unavoidable magnetic materials, but, if you use them, be sure they are consistent with the readings from a GPS or other nonmagnetic direction-finding system. Remember that GPS will not give you a reliable compass bearing unless you are moving, and you have a GPS display that shows a compass on a screen.
Two major kinds of cables will be on fishing vessels and larger recreational boats: 12 volt DC and 120 volt AC. There may be a few additional kinds, such as high-voltage cables for radar and other transmitters, and short low-voltage cables for electronics such as 5 and 9 volt DC, but the twelve and 120 will be the common ones.
You want to keep high-frequency signals, which can be from obvious sources such as radio and radar transmitters, but also from more obscure ones such as data cables that send individual bits and high speed, from coupling to the power lines. Power lines can inadvertently carry high-frequency signals to equipment anywhere in the boat. Be aware that RF can travel through power grounds
If the interference can be traced to power lines, it may be useful for a qualified electrician (for cables carry dangerous voltages) to install a radio frequency choke, which is a component that "drains off" the high-frequency signals without disturbing main power.