Circuit closing or return to source??

[Coyote24]

Quick question.
" Most of the connections between the Raspberry Pi, the MacArthur HAT and your boat are not isolated. To avoid damaging the Raspberry Pi or MacArthur HAT, it is therefore important that everyone agrees on what represents ground, to which all...
To ensure this, make sure that there is always a connection from the MacArthur HAT to the boat ground"
Is that saying to return to the the battery NEG post (to close the circuit) or the boat NEG ground, usually the engine block, so the boat electrical potential is balanced??
I think the first.

a.

[Adrian]

Quick question.
" Most of the connections between the Raspberry Pi, the MacArthur HAT and your boat are not isolated. To avoid damaging the Raspberry Pi or MacArthur HAT, it is therefore important that everyone agrees on what represents ground, to which all...
To ensure this, make sure that there is always a connection from the MacArthur HAT to the boat ground"
Is that saying to return to the the battery NEG post (to close the circuit) or the boat NEG ground, usually the engine block, so the boat electrical potential is balanced??
I think the first.

a.

Yes, ground in the documentation is referring to electrical ground, i.e. the negative terminal of the battery.