I speak dutch but i'll keep it in english
In my oppinion it is rediculous that in Holland you need a different piece of paper than you need here, but in practice it is the same! however, laws are laws, and up on the maas (binnenvaart) the police make quite a bit of checks around. Out on sea i dont think you require any license, or do you.. i moved away before i got into into boating.
Anyway, best way round the RYA stuff for Croatia is probably taking a nice week on holiday out to Wales, or scotland or whatever you want to do, grab an easyjet ticket at £30,- both ways (or €45 ish) and go and sit in a Bed& breakfast for a while for about £10 a night, get 2 days training, get the Level 2 powerboat thing. Enjoy the rest of your holiday, and head home.. nice little relaxing holiday! (i believe the level 2 is around 170 quid, but cant remember as the Highland enterprises paid for mine as a skills for the community project)
okay.. hope i gave some ideas...