Last May a group of us chartered a pair of Jeanneau 37's from Euro Agent.
Two skippers had their Yachtmster Offshore Certs available. Nobody asked to see them. Nobody asked for anything other than a credit card for security deposit.
Charter company wanted a copy of my dazed kipper cert and my VHF licence. I had chartered with them (Turkey) once before. I was never asked by Croatian authorities for any paperwork.
Apparently the authorities do want some kind of skipper certificate. I don't have any at all, so the charter company, very obligingly, forged one for me! They definitely got my vote for that one.