50 or so years ago, auto manufacturers switched from 6 volt to 12 volt systems in cars and light trucks. The advantages were that smaller gauge cables could be used, saving cost and weight, less voltage drop from long cables and connections, and smaller and less expensive motors.
A 24 volt electrical system in a boat wuld have the same advantages over a 12 volt system. But:
24 volt boat parts are pretty uncommon. Go to your favorite marine store and look for 24 volt bulbs, pumps, radios, etc. Chances are, you will find few or none. Parts will be special order and the selection will be small and the price high.
Basically, you pay your money and take your choice, but I think the standard 12 volt system is the most practical until you approach 40 or 50 feet in boat length.
A friend of mine was called to work on a boat with a 32 volt electrical system. You won't find those parts at your local West Marine store.