This early Military Policy is only valuable if you are a seafaring civilization with access to the sea, and a desire to develop a navy. In this case, Maritime Industries provides a whopping 100% boost to all earlier ships! However, even then you shouldn't keep the Policy in your government permanently, but only until you finish building your navy. As usual, try to concentrate on that with all cities capable of building ships, so that you can replace the Policy with something more useful.
If you are in no particular rush to conquer the seas, you shouldn't use this Policy at all. There are many better alternatives to your early Military Policies, which will not only save you valuable time, but also expand your other gameplay opportunities.
Civilopedia entry Edit
In some ancient places, men took to the seas, exploring, fishing, trading, and fighting. And in some of those places, maritime industries – the construction and outfitting of ships – haphazardly arose, making these men somewhat better at this exploring, fishing, trading, and fighting. Although the Egyptians built the first sea-going ships, the Minoans were the first great seafarers of history, trading as far away as Sicily while their King Minos conquered the other Aegean islands. The Phoenicians built a maritime “empire” of loosely-allied city-states. In far India, the Mauryan Empire (4th Century BC) developed the earliest known state organization for shipbuilding, and “invented” the science of navigation.