What gets installed
Chartplotters, fishfinders and transducers, VHF radios and antennas, stereo and speakers, courtesy and underwater lighting, additional batteries, switches and charging systems, bilge pumps, and the trim and gauge accessories that get added a season or two after the boat was bought.
Doing it so it survives the winter
Marine-grade wire, correctly sized for the run. A fuse or breaker at the source, not at the device. Terminations that are sealed. Transducer placement thought about before a hole is cut. Cable runs that are supported so vibration does not chafe them through against a stringer over three seasons.
The standards that govern this on recreational boats are published by the American Boat and Yacht Council, and they exist because the failure modes are well known and entirely avoidable.
Where the power comes from
The single most common defect in cheaply-fitted marine electronics is not the device, the mount or the wiring gauge. It is the source. A plotter, a stereo amplifier or a set of underwater lights spliced into whatever circuit happened to be nearby works on the trailer and misbehaves at cruise, when the alternator, the bilge pump and everything else are drawing at once.
Doing it properly means a feed sized for the load, taken from a sensible distribution point, protected by a fuse or breaker at the source rather than at the device, and with a ground path as deliberate as the positive one. On boats with more than one battery it also means deciding which bank the accessory should live on, which is a question most kits never raise.
The result is an installation that behaves the same in August as it did the day it went in, and does not quietly flatten a starting battery overnight at a mooring.