|   There's a lot to be said for having some serious thrust available for several hours straight.

Is there not more to be said for having a safer designed boat to start with?

How many hours? How much thrust? All of this will be a probabilities/ personal comfort assessment, right?

I am not enough of a sailor to suggest a critique of your given example. But I am not sure the best lesson learned is MORE POWER! Are you sure that it is?

|   You may never need it, but that said, most cruisers I know have at least one or two stories to tell, often involving a boat getting dragged in a crowded anchorage (either their boat or another).

That's an interesting point. Perhaps the answer is always have more power on tap.

|   But...  I don't think I'd go with lithium batteries on a cruiser that leaves sight of land.
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|   Even though the LiFePO batteries are more stable than lithium-ion, they still burn, and they don't go out until the boat is submerged. Which is a problem if you're depending upon that boat. 

Firstly, the risk of a lithium battery fire is very small. Especially a bespoke hand made and well instrumented system like you or I would build.

Second, the best way to put out a lithium fire is water, because cooling is the most important thing. So, I disagree. It is rather easy to stop a lithium fire on a boat, cause water.

Epoxy and foam burn, and they won't stop once started.
Catching lithium on fire is, perhaps, about as easy or hard as catching epoxy and foam on fire.

Keeping lithium batteries cool is stupid easy on a boat surrounded by water and with super cheap battery monitoring chips, it is cheap, light, and easy to know the exact health/state of your battery packs.

I am not trying to debate your personal comfort level. I am suggesting your personal concerns seem a bit miscalibrated to me.

| For a Tesla on the highway, you can step away and get picked up by AAA.  For a boat in the middle of the ocean, or a crowded anchorage, the situation becomes more complex.

One builds faith and understanding in the technology through safe exposure over time. Yes, do not rush to cross an ocean in a new bespoke boat and power system. Try it close to land first, perhaps.

But, I bet, in the final history of man, more boats will be lost to out of control hydrocarbon fires than will ever be lost to lithium battery fires.

|   I /really/ want to skip the internal combustion motor altogether, but for now, there's not really a good substitute.

I agree. But the gap is closing and fast. As I eye the time to decide the power system for my boat, and the closing gap, I am pretty sure the gap will be well closed before I must decide.

But I am often wrong.