To start with a digression, in the middle ages the highest "rank" on board a commercial vessel was classically considered to be the Master, not a Captain. Term Captain on ships was used for the commander of soldiers on board - usually a noble - and having soldiers on board would likely never happen on commercial ventures but rather on war campaigns. In theory even when the ship carried both, it was the Master who was responsible for all things nautical, from navigating and piloting the ship to managing the crew, taking care of supplies and all things we generally associate with captain-buisness. In that time the Captain was suppose to be staying out of the way. However the Captain could - especially in the military operations - override the Master on what to do, where to go, and how to proceed, basically outranking him on the nature of both his higher social status but more so the fact that the Captain was often connected to the organizers of the expedition, sort of their representative and extension of authority, be the organizers the Crown, State or private individuals. Similarly how if for example the merchants charted the ship they couldn't run it and do what they want, but they can give general orders to the Master of the ship, even if he owned the vessel - if the orders followed the law of course.

With the increased number of state- and noble-owned shipping and the participation of nobles in those, the line between responsibilities of Captain and Master became harder to distinguish, until at some point they merged and it became mandatory for Navy ships to always have a Captain as the highest rank. I am not sure if even at that point commercial vessels did without the captain, and only master? In each case, we got the currently accepted vision of a Captain being the person in charge of a ship and boat.

All the above isn't just to clarify a false premise, but to introduce a great resource for everyday stuff in medieval shipping, a book The World of the Medieval Shipmaster: Law, Business and the Sea, c.1350-c.1450 by Robin Ward which focuses on duties and legal responsibility of the Master in Late Medieval times, as the main person responsible (and often the owner or part-owner) of a commercial ship. He operated in a set of complicated junction of international maritime and local civil laws and jurisdictions, where many things were actually quite well - albeit hard-to-enforce - regulated.

Now, food and rationing was one of those, although not enough to answer your question just on the basis on the laws. One set of maritime laws, from Oleron, as presented in the book, ordered that ships rations had to provide either one hot/cooked meal and drink (wine, or for England beer) or if no drink is available, two cooked meals per day. The commodities are not set, but the book provides some examples. Inability for preservation of food for longer time dictated what could be used. Fresh foodstuff, especially fruits, vegetables, even bread was limited only to when in harbor and few days afterwards. If the ship was to be longer out of reach of ports (which to be honest was less a problem in Middle Ages then later in times of transoceanic travels) it came down to dried, salted and smoked fish and meat, as well as hardtack biscuits and longer lasting double baked breads. Larger ships could have their own ovens, and fresh bread could be baked from easier to store flour/grain. Nuts of different sorts and beans,leaks and such vegetables could also be used, although they came in lesser proportions. Wine was used in French, Spanish and other southern European ships, but English used beer as an substitute when wine was unavailable.

From the above book here is a quote of what one ship in 1440-41 bought for the preparation of an ordinary voyage:

In the owner’s accounts for the preparation of the Christofer of Dartmouth for a voyage to Guyenne with Edward Hull in 1440–41, the victuals bought consisted of wheat flour, a small quantity of oatmeal, beef carcasses, three different types of salt fish and beer, the quantities indicating that all ranks, crew and retinue, were to share the rations. The accounts may be incomplete, in that last-minute purchases were probably made of fruit, vegetables and dairy produce.

The author also gives a breakdown of victuals for the same ship, along another ship named Margaret Cely from 1488-89 of percentage of victuals by value, which i provide here. The difference in percentage of some victuals e.g. fish even for the same ship but different voyage the author attributes to different seasons, food availability and prices, and of course hope to be able to catch fish so less was bought.

For further reading on food rations on ships of Age of Exploration, I gave an old answer here and on fishing here