I don't know about the rest of the EU, but I can tell you the situation here in the UK:
Summary: for strictly recreational purposes there is no legal requirement. If you had your own boat you could take it out with no training, experience or certification at all (thankfully most people don't). So the level of certification/experience needed depends on the people who will be hiring you the boat (more accurately, depends on their insurers).
Inland waterways (rivers, canals, lakes, lochs, reservoirs, Norfolk broads): you don't need anything at all to hire a dinghy or a cabin cruiser. (Sometimes single-gender adult-only parties are banned because they don't want stag/hen groups, but that's a different issue). (The major booking agents are Hoseasons and Blakes but they don't actually own the boats themselves, they take bookings for a wide range of small boatyards and in these internet-enabled disintermediated days you might find it cheaper to go direct to the individual boatyard).
Offshore: it varies depending on the charter company, the size and value of the boat, and the area where you intend to sail. It's not unusual for a charter company to restrict you to a limited area (eg the Solent) if you have less experience, and allow you to go further offshore or out into a less sheltered area if you have more experience.
So the thing to do is for you to email charter companies and see what they say.
One of the major charter companies in the uk is Sunsail; their experience requirements are here:
For sailing out of their UK base in the Solent, the experience they're looking for is:
- 20 days or 400 miles as skipper on an equivalent size yacht
- RYA Day Skipper with experience to a higher level
- RYA Coastal Skipper
- ICC with experience to a higher level
so if you have a sailing cv you can show them with a decent amount of experience, then that plus the ICC should be enough.
Technically you should also have a VHF radio certificate in order to be able to use the VHF legally, but this doesn't seem to be enforced unless people start misusing the radio, and the charter companies don't seem to care.
What operator certificate do I need?
You must hold at least the Short Range Certificate to use hand held VHF DSC, just like any VHF DSC radio.
Powerboats on inland waterways: on larger rivers (eg the Thames) you can hire a RIB, although (on the Thames) there are speed limits until you get
pretty far downstream. For these boats the situation is the same as for offshore charter boats except that the qualification the company will usually be looking for is RYA powerboat coxswain level 2. You might be able to convince them that a different Australian qualification is equivalent - contact them ahead of time as they may need to check with their insurers.