Boatsurgeon, I can't answer to all of your statements, so I'll just hit the ones that I can.
1-4) Arc distance for 48V is .6mm. While that's bigger than 12V's .1mm, I can't see any situation where bus bars or terminals could be placed that close together, even if you wanted to. If your concern is dropping a tool into the battery box, I have you covered - I'm putting terminal caps on all of my battery connections so they'll all be insulated.
5) Definitely. When the time comes to actually connect the terminals in my battery box, I will be wearing rubber gloves. While I've been poked occasionally with 120VAC, I don't enjoy the feeling.
6) More likely I would bump my head on the bulkhead of the boat, but point taken.
7) Maybe? Thing is with things like inverters, you get what you pay for. A quick look shows that 48V inverters are marginally (+5%ish) more expensive than 12V inverters of the same power rating.
12) Yes, it does give you a single point of failure. However, for the cost of a 12V battery, charger, and separate wiring, I can buy a dozen 48V to 12V converters, all with an IP67 rating, eliminating the need for a house battery set. Even with a dozen spares in the hold I'm still way ahead in the weight of a single medium sized lead acid batter and all the accoutrements that go with it. This is one of the few times on a boat where it's easier, cheaper and lighter all at once.
14) It's more than justified - it's essential. At 1V = 50 RPM, a 12V motor would get you 600RPM. That might work for a dinghy, but it sure won't push a sailboat at better than a crawl, no matter how many amps you throw at it.
17) Yep, that's me. But as the old 60's Easter special song goes...