Boats and electrics don't seem to be a good mix: here is my experience of failures:
I have had "cabin cruisers" with an enclosed wheelhouse since 1989. Items exposed to the exterior are often short-lived; but even items mounted in the wheelhouse don't last long. I am fairly confident about electrics, so try to mend things that go wrong.
Horns Most probably cheaper electrical horns are automobile specified and so don't last on the cabin roof. I have had 3 "shorty" electrical horns fail, each after about 3 years. One I replaced with an air horn [electrical air pump inside cabin and only air pipe and plastic horn outside]. Another I replaced with a "nautical" long electrical horn which has lasted better- though it has now (2023) died - after 15 or so years of service.
BMW outdrive leg position sender. This was mounted at the side of the outdrive leg and failed. I attempted to repair it but not reliably. The failure was costly in that I started the motor with the leg up - damaging the bellows.
VHF Aerial. Mounted above the cabin roof, I have had two fail. I now fit a spare aerial as well to cope with such a loss.
Navigation Lights. The traditional sort has special
bayonet bulbs and the contact can fail. This can be cleaned and repaired.
The green navigation light had a "disappearing" green layer inside -- so
that it showed white. After a repair using green marker pen [short - lived],
I had to replace the light completely. I have subsequently fitted a
green LED light (it is much more efficient to generate green rather than generate
white and filter out all but green).
The red navigation light has also shown a fading of the red filter
after 20 years - so replaced by a red LED bulb.
Also anchor light failed - corroded - so whole unit replaced.
Spot Light. The steerable spotlight on the wheelhouse roof has a halogen sealed beam light which failed after a few years. The Jabsco 135SL spare part is over £75 - an expensive light bulb. I found that you can order the same part from the USA directly (it is used in spotlights on off-road vehicles, H9405, made by Wagner) for a much lower price (circa £25). Replacement works fine.
Radar. See Furuno below.
Bilge Pumps. Electrical bilge pumps located in the sump below the engine can fail. I have had two fail. Two float switches have also failed. Another 2 pumps (self priming so located higher up) also failed.
Exhaust Alarm This lives on the exhaust tube (big corrugated
rubber pipe) and sounds a buzzer (and has a red light) in the wheelhouse
if the temperature is too high. The first one had brass contacts which
corroded and snapped off after 6 years; the replacement gave erratic
false alarms because of a build-up of salt on the terminals and then
failed as terminals corroded and broke after 13 years.
The alarm display (located on the wheelhouse console) had its
red light (24v mini filament bulb) fail after 15+ years (despite almost
never being on). Replace by red LED protected by suitable resistor.
Then replaced by a home-brew exhaust
temperaure gauge and alarm.
Smoke Alarm I fitted a battery-operated optical smoke alarm in the engine compartment. This worked OK for a few years and then began to give false alarms.
Relay Engine start relay (located near starter motor to provide current to solenoid) failed. You can start it in an emergency by bridging the terminals with a metal spanner.
Starter My starter failed after 13 years - return spring shredded - replaced (heavy item - 7.8kW rated) - and old one reconditioned and carried as a spare.
Alternator After problems with the alternator, I now carry a spare alternator so that I can swap it in should problems recur.
Engine electrics. My current engine (Perkins-Sabre M215C) will run with NO electrics -- provided you can get it started. If there is no electric power, the stop solenoid won't work; so you have to move a lever to stop it. And, of course, the diagnostics (revs, temperature and oil pressure) won't work. The engine is said to continue running as sea-water rises around it until the air inlet is below water level!
Fuel tank level sender. Failed - giving spurious readings. Repaired.
Engine display console
Voltmeter (VDO) mis-read after 1 year from new: annually
replaced under warranty several times until I got fed up and fixed it
myself. It had been modified for 24 v use by adding a parallel resistor
with a variable option. This variable resistor had its contacts get
corroded after about 1 year and went open circuit: causing the voltmeter
to read high. I replaced it with a suitable fixed resistor soldered in:
problem sorted.
The hour-meter lost its LCD display (a known problem again caused
by contacts corroded). I fixed this with a new separate
electro-mechanical hour-meter -
which only lasted a few years - so replaced with an LCD model.
The rev meter is also erratic.
some failures:
24 volt kettle died; replaced; died [relay fault]; replaced
Fluorescent cabin light died
solar panel (1 of two); repaired
fridge fan motor replaced
24 v to 12 v dropper [resistance type] died; repaired
24 v to 12 v dropper [switch mode] died; replaced; died; replaced.
diesel heater motor failed; repaired by dealer
wheelhouse VHF erratic (dual watch not working properly)
Hand-held VHF died
Electric motor of autopilot hydraulic pump failed (after 19 years service)
AM radio (old 12 volt car type) failed - repaired
Switch/fuse panels have toggle switches which seem to be rated at 3-5
amp. One switch failed [stuck on - which trashed impellor] on one of the
bilge pumps [7 amp drawn]. Replaced using a relay to lessen stress on switch.
Furuno This company has a reputation for "bullet-proof" kit and mine have indeed lasted well. My GPS (GP35) failed after 13 years when the contrast of the LCD screen became unreadable. My radar (1622) failed after 16 years when the scanner unit electrics gave up. [I had also had to reseat the drive belt in the scanner a few times during its life].
Dead on arrival:
I have ordered various new equipment that has been not OK on
arrival. In each case it was replaced under warranty - but it would have
been better to avoid that hassle. Note a common theme (since Apelco was
linked to Raytheon that span off Raymarine...)
I bought an Apelco GPS in the USA long ago and tested it there.
When I used it in the UK; it decided my location was 99° South.
This was hilarious: a location that is not just wrong but cannot exist.
Apparently the US model couldn't cope with longitude near zero!
The Raymarine ST5000+ autopilot came with a non-functioning
relay that drove the electric hydraulic pump.
The Raymarine RC435 chart plotter had defective NMEA output.
On the plus side, once working, this autopilot and chart plotter have both lasted well.