If your boat comes with 24v as ours did, or it's an option on a new boat, it's great.
Reduced current means less voltage drop. "Everything is brighter and works better" is the best I can describe it, e.g. the 24v toilet has a lot of grunt!
However, some items can be a little hard to find - some light bulbs for example, and you may need*a 12v convertor in order to run a VHF radio,*car stereo*or laptop.
I certainly wouldn't convert to 24v from 12v, or let it influence a buying decision unduly, but would definitely specify 24v on a new build.