Gardes de la marine

In France, under the Ancien Régime, the Gardes de la Marine (Guards of the Navy), or Gardes-Marine were young gentlemen picked and maintained by the king in his harbours to learn the navy service, and to train to be officers. They were organized in companies, divided up between the harbors of Brest, Toulon, and Rochefort. All naval officers were drawn from these companies, which were the equivalent of the current naval school.

The king paid schoolmasters to instruct them in everything they needed to know to be good officers - there were masters in mathematics, drawing, writing, fortification, naval architecture and construction,dance, hydrography, fencing etc.

They sailed on the king's ships, on which they served as soldiers, and acted in all roles on board. Undergoing further training at sea, they honed the skills they had learned ashore. Their orders, in cooperation with the captain of the vessel, included four hours intended for their different exercises. The first hour was in piloting and hydrography, the second for musketry and military manoeuvres, the third for cannon exercise, the fourth one for training in steering a ship, if time allowed, supervised by the captain or second in command, done by each of the gardes in turn.

...
Wikipedia