Generally all very excellent views that I share, but to illuminate a little more on some of them - -
Neatness goes a very long way towards a safe and efficient systems. That is, a single battery
sized cable to the engine block from either the "load-side" terminal of the shunt or the engine start battery itself is better than lots of negative (ground) cables
from here, there and everywhere.
In order for the engine starter and the alternator
and the engine instruments to function the engine block needs to be attached to the boat's DC negative (ground) system.
And getting rid of the "rat's nest" by either starting over with proper marine-rated electrical
wire or at least sorting out and arranging existing wires into logical and neat groups is desirable.
Whether the boat's DC system is or is not connected (grounded) to seawater is usually a moot question as the design of the engine propulsion
system in the vast majority of boats results in metal to metal attachments that end up with the propeller
which normally is immersed in water
. However, a few boats have propulsion
systems that are "isolated" electrically to reduce or prevent electrolytic deterioration of the drive system. Again, the voltages in an average boat's DC system are not high enough to pose a hazard to humans so sea water
grounding is not a major consideration compared to a boat's AC system.
It is rare these days to find DC powered instruments/loads that have metal cases, most are plastic. But in the ancient DC systems (my old boat had quite a few) metal cases were indeed bonded to the DC negative terminal of the instrument/load so that a positive lead short to the metal case would trip the fuse/circuit breaker rather than pose a spark or fire hazard.
GFI's are now (in the USA, at least) a part of the ABYC guidelines and without proper bonding between genset or inverter
neutrals and safety
grounds, the GFI's would constantly trip or refuse to stay set. AC "leakage" was rampant in the old equipment
and a real pain to correct.
I suspect it was primarily a cost issue for boat builders to simply bond the AC safety
ground to the DC negative (ground) system rather than installing a separate sea water ground plate for the AC system. Here again, the voltages in a normal boat's AC system can do serious damage to humans so having the system be able to trip the circuit breaker or GFI when a sufficiently large AC leakage developed was necessary.
Personally, I prefer keeping the boat's DC systems separate and apart from the boat's AC system. In AC to DC bonded systems frequently AC "leakage" would cause havoc with sensitive instruments not to mention that "tingly" feeling whenever you touched a metal part of the boat especially after climbing back onboard after a swim in the ocean.