In general this will be at the discretion of the boat owner, although some European countries do require an ICC certificate (equivalent to RYA Day Skipper) for boats over a certain size. An RYA Day Skipper certificate is usually a safe bet for most boats; or submit a sailing CV if you’ve done a lot of boating but never taken this out. Smaller boats will need less experience, but boat owners will usually like to know that you’ve at least handled a boat before.
If in doubt ask the boat owner, and there’s always the option of booking a skipper for the trip. This has its own benefits and will allow you to kick back with your family and friends; relax into the sail whilst someone else gets you from A to B. Hosts are also available for meal provisioning, and full crews for larger boats. See the optional extras on each individual boat listing for what’s available.