question

I went through (and still am I guess) a fair bit of analysis, looking at various approaches etc before kinda maybe committing to open ended solutions :D

I think it really depends on the hypothetical scenario you're facing and the strength of the threat.

A (significant) direct strike imo is more about managing the risk of fire, electrocution and loss of structure. The induced currents as a result of diverting a strike through the downconductor is highly likely (again imo) to bypass almost any electronic protection that you might have simply because of the surge magnitude and physical proximity.

Indirect strikes on the other hand, you may have a better chance because the magnitudes are lower.

The failure scenarios I'm thinking of are:

1) Surge at input of DC/DC is stopped at DC/DC

2) Surge at input of DC/DC blows through and is present at output with a reduced magnitude

3) Surge is coupled to output (through environment external to DC/DC)

The 3rd one is a doozy because of physical proximity and conflict with functional needs.

If you have a depth sounder, does the transducer look like a small earthing device wired to the 12v system? The potentials involved in a direct strike is not going to blink at the DC/DC's isolation rating.

More broadly, I think of lightning protection on a boat as needing layered defences, including physical separation, isolation, diversity, modularity, etc. An isolated DC/DC converter is one single layer - not a bad one but I wouldn't go so far as to say its sufficient on its own.