It's definitely an interesting idea and wouldn't be that hard to pull off. Doing it during a major rebuild would make sense. The dc/dc converter would have to have built in voltage regulation but after a brief search it looks like most do (~$260), a simple step down transformer wouldn't work. Being able to go a little smaller on all of your wiring like you already mentioned could provide you with some pretty decent savings as copper is so expensive these days. It may still be a good idea to run a 12v circuit up to the dash just for devices that may not be able to handle 24v (depth sounders, gps posibly) and a couple of 12v outlets
Resale might be effected a bit as it may confuse your average buyer but if this is a project boat you probably aren't too worried about that.
I think another interesting project would be adding a belt drive water pump with a magnetic clutch to run off of the engine, you could have one heck of a ballast sytem build around that baby.