Who can rent a boat? Everybody ! That said, if you do not have a license or nautical experience, you will need the services of a skipper, formerly called a "skipper". The latter will be the helmsman of your sailboat or the captain of your pleasure boat. The owner of the boat or the company to which you rent your boat will be responsible for finding you this ship's companion.
And abroad ?
You will need to obtain the International Certificate of Pleasure Craft Operator (ICC, also called ICC); a simple translation of your national license!
That said, it is not necessary to obtain it if you hold a pleasure boat license of a state of the European Union to navigate in the coastal waters of the Mediterranean countries and the European countries of Europe. However, if it is a rental of a sailing boat, no need for a license but you will need to provide a nautical CV attesting to your experience of skipper at sea and your indoor rentals. In all cases, to rent a pleasure boat, you will be asked for an ID, a list of crew and sign a rental agreement!
Flag of the boat, what does the regulation say?
The regulation of the flag implies that "ships sail under the flag of a single State and are subject, except in exceptional cases expressly provided for by international treaties or by the convention, to its exclusive jurisdiction on the high seas".
So, let's simplify things a bit! It is therefore the nationality of the ship that determines the legal status, not the nationality of the owner or crew. In short, if you sail on a French boat abroad, it is the French regulations that apply but if you sail on a foreign boat, you will have to refer to the regulations of the country in question.