To rent a motorboat without a skipper in France, you must hold a boating licence if its power exceeds 6 horsepower.
For renting a sailboat in France, no licence is required. However, some professional lessors may occasionally ask for it for insurance purposes.
If you don't have a licence, you can still rent a boat with a captain (skipper). They will be responsible for piloting the boat and taking you to the best navigation spots.
For renting a boat abroad, you can check here or inquire on the website of the maritime authorities of each country.