Just a couple of points from my own experience on some things raised -
Insulation of metal boats - should be regarded as just part of the requirements of building them, I cannot imagine a metal cruising yacht built without insulation as it will be like a drum (and I would insulate a frp cruising boat for myself too if not foam sandwich). Flexible closed cell insulation sheets
glued onto the plating is widely used in superyachts (eg Armourflex, Korex) and 1 inch of that will result in a boat quieter than other constructions. Our own steel yacht with 1 inch of Korex was quieter than the builder's own timber composite yacht and I also stand by Noelex 77's comments related to not being able to hear things outside the boat or, indeed, the other way around. We do hear water
"sloshing sounds" when sailing but only from our own water tanks
- such boats can be uncannily quiet.
A comment was made about the failure of aluminium on high speed ferries (which I have managed the design and construction of). This is not a fault of the aluminium as a material but rather of the need for light scantlings in such vessels so that the boat will perform. While these vessels are mostly built to be in class there is considerable difference in the scantling sizes allowed between the major classification societies eg a boat designed to DNV rules will be considerably lighter than one built to Lloyds. But in the end, the striving for low displacement
to enable these vessels to be fast and still stay a modicomly frugal in fuel
use is the problem rather than the material (I've known of big fast ferries that are dived on weekly in order to check for cracks in the plating). No other economic material would allow them to be built to such light weight budgets.
Painting of aluminium has been mentioned as a problem. Painting aluminium is not a problem these days. In the past (maybe 15 years or more ago) there were difficulties with poor marine
alloys and a poorly designed more modern boat may have difficulties in some areas - if the paint
won't stick then something is wrong with the paint or the alloy. The preparation I see now on aluminium boats is generally simple coarse sanding
of the alloy plating followed by application of an epoxy primer, followed later by fairing and build coats, then polyurethane
topcoats. Only one example - I have an aluminium commercial
vessel which is more than just a few years old parked next to me and there is no white powder to be seen anywhere on it, neither on the painted parts
or the unpainted parts
(whether unpainted intentionally eg rails, or unintentionally due to accident).
Fairing has been mentioned as falling off. I have not heard of this myself for decades. In the good old days fairing was done with polyester, which was a problem in that way. For decades fairing has been done with epoxy incorporating fillers and have never known of any problem with it falling off, although like anything a bad job is a bad job and may lead to problems.
Note that my making these comments should not be just assumed as my being married to steel or aluminium - I did say earlier that my first choice would be a very expensive exotic laminate construction
Lastly, has been mentioned that steel and aluminium are environmentally unfriendly from a carbon footprint point of view. Putting aside comparisons with other materials, I expect that in the end one's view depends as to whether one is a watermelon or not ("watermelon" - green on the outside and red to the core