You can’t possibly know what you need until you complete a full inventory of what you already have. Pull out all your bins, clean out the bilges, and dig under the couches and beds to see exactly what you have on board.
Then tackle your storage room. Be sure to determine the age of spare parts, supplies and filters to see if they’re still usable. When you reach for something in a pinch or an emergency, it has to work, so when in doubt, throw it out.
Once you have a handle on what you do have, go through each of the ship’s systems to determine which spares you need. Taking a picture with your phone—noting part and serial numbers—is a great way to inventory everything on the boat, and it’s much easier to flip through photos than trying to make a list.