I don't have an experience as waiter or bar tender but I will answer you with confidence that
YES it is really easy to find a casual job such as waiter or bar tender. The main reason is that Greek Economy is based on tourism sector and also North Europe such as Greece love the sun and want to go often for a coffee or a beer.
In these years of Economic Crisis 2 things have grown here in Greece lottery and catering businesses such as coffee shops hotels etc.
Also Economic Crisis is a little a bit on our minds there are a lot of jobs in Tech sector.
Main problems you will have:
1. Yes greek language maybe is a problem but greek people know english especially young people
2. Another problem is that if you work in a coffee shop maybe you dont have insurance for your pension here in Greece and this will be problem only if you would like in the future to be a Greek Citizen.
3. You should be ready to work hard. The main problem here in Greece isnt Economic Crisis but micromanagement as people who dont have the skills to manage people.
But you should always remember Greek people are happy people in most of the cases will help you and also Hospitals are free to everyone if you are European citizen with European Medical Card something like that.
Maybe money aren't so much but compare to prices are fair if you work hard.
A good startup for your needs to find a casual job is https://www.myjobnow.com but the problem is that website is on greek language so talk to the chat