This is the list of documents that we brought with us on our trip, the list will most likely grow in the future but till now it works as a pretty good skeleton for us.
Certificates related to yachting
European health insurance card (Europeiska sjukförsäkringskortet)
Extended travel insurance documents
Purchasing contract (with VAT included)
International certificate (the yellow book from SXK)
Manuals for more or less everything on the boat.