Can't answer for every feudal monarchy, but, at the outset of the HYW, neither France nor England (arguably the most advanced feudal states in Europe) had a standing, professional fleet. The English monarchy owned one single warship (which was quite old and unfit for service). France, which was considerably wealthier and more populated, had a dozen war cogs, but most were several decades old.

In the early 14th century, the general idea of naval warfare in this part of the world was to requisition civilian and trade ships and to load a bunch of soldiers/archers onto them. Which is what both the English and the French did at the battle of Sluys. The French also regularly hired Genoese galleys and sailors, regarded as the best you could afford back then (and sometimes "forgot" to pay them).

Things changed throughout the conflict, as both sides (and their respective allies, especially Castille) tended to build more and more ships designed for war, and as gunpowder weapons became more and more widespread. Charles V of France, for example, built many shipyards, while the English bolstered the Cinque Ports (the "Five Ports") capacity. But even though some other large scale battles (La Rochelle, Arnemuiden) happened, most naval warfare involved piracy and raiding coastlines with refitted trade vessels.

By the end of the war, the English monarchy was ruined, and couldn't afford a professional fleet, and the French, for obvious reasons, favored their artillery over their navy.

Note that the idea of feudalism and of a fleet are somewhat contradictory. Building big war vessels requires loads of money, which is something only a functionning, centralized state can provide.

Sources: La guerre de cent ans: naissance de deux nations, Georges Minois.

Le temps de la guerre de Cent Ans (1328-1453), Boris Bove