This is basically a "Common Ground" question. There are two approaches,
and they are both workable.. depending on which is more reasonable on
your particular boat, and it's configuration and wiring access.

First, if you're building or rebuilding a boat, I strongly recommend that
you install a run of plastic (Typically PVC) electrical conduit along one
side of the boat (typically the right if a right-wheel powerboat),
located just below deck along the sheer. Run this from stem to stern, and
size it bigger than you think you need! I last used 1 inch ID and it wish
I went to 1 1/2" ID. At each useful location, such as electrical panel,
engine, bulkhead etc, either install a junction/pull box or simply cut
away about 90 degrees of the conduit for an inch or so. It's easy to
install wiring by pulling it thru the conduit, and a stiff metal 'snake'
can be pushed from one location to another to add wiring later. You will
add things you never thought of at the beginning. This is just a
mechanical way of taming the "Rats Nest" seen in some boats. Now to the
grounding issue.

I am in favor of running a LARGE (Say #00 cable) ground cable from stem
to stern, that is well bonded to the rudder shaft, the propshaft and
stuffing box, the engine block (also battery negative common point),
hull seawater grounding plate(s) if used, the mast base (if sail), the
"metal above the head" such as windshield frame, top-mounted antennae
bases, radar, weather instruments, stays, safety rails, anchor chains,
etc. I have run rubber covered welding cable, and also plastic-covered
power cable for this ground, and the routing of it is a tradeoff. It
would be nice, electrically, and lightning-related, to run it right down
the center bilge, and I've done that, but had some corrosion problems
after 10 years or so. Last boat I ran it along the chine, thru holes
drilled in the frame, with a major side tap to the power panel area. IF
that had been a sail, I'd have run a large tap down (Or over) to the mast
base.

But, back to Grounding. The two main choices a Run all grounds to the
nearest tap on the one large grounded conductor (Common Ground), or (Star
Connection) run separate power and ground conductors from every load back
to the power panel area, and a single ground point there. I have chosen
a compromise: Larger loads go directly to the large ground conductor, and
only send the power feed from the power panel. Smaller miscellaneous
lighting, horn, etc have two (typically twisted) wires run from the power
panel. Simply easier to wire.

What are the tradeoffs? Here's what I see (comments and experiences
desired):
(1) Voltage drop: Minimize voltage drop in ground circuits by fairly
direct connection to large low-resistance ground conductor.
(2) Size / ease of wiring: two wires easy for small stuff, single feed /
less wire for large stuff.
(3) Electrolytic corrosion: There are two concerns: (A) circuits with +12
or +24 on them HAVE to be well insulated and dry or corrosion will occur.
(B)"Ground" connections that may well have dampness and salt near or on
them need to have the lowest practical resistance between them, and the
least constant current through them, to minimize voltage drop which
causes electrolytic corrosion.

The high-current paths (battery-alternator-starter) should obviously be
separate from the rest of the boat systems, and well insulated and
protected from dampness and corrosion. They get connected the the rest
of the boat in two places: Battery negative/engine block to the boat
grounding system discussed above, and battery positive (typically from
battery switch(es)) to power panel.

I'm pretty amazed at the number of boats that have major items like
masts, stays and rudders ungrounded...

Comments appreciated; there's more than one way to skin a ground
conductor.
--
Regards, Terry King ...In The Woods In Vermont

The one who Dies With The Most Parts LOSES!! What do you need?