Lightning protection systems and marine SSB radio systems need a direct
connection to the sea water. DC electrical power systems do not need a
connection to the water. Just use a common grounding point for the negative
side of the battery (or batteries) and the negative leads from your
electrical loads. Blue Sea makes some nice ones with large bolts for the
battery leads and smaller screws for the various loads. You could also creat
your own from a strip of copper plate and either use bolts and nuts or
threaded holes for the connections. Some MaryKate Tek Gel assembly lube will
help protect against corrosion.

By the way, plastic thru-hulls fittings are not recommended for wood hulls.
The expansion and contraction can crack them. You really should use bronze.
That advice also applies to thru-hulls for speed and depth transducers.

Rusty O