I think that much of this could be in interpetaion of of contract. Could be a language issue, could be a great con, who knows without first hand experience. Unfortunately, when we get into things Greek on this site it usually feels like competing Greek captains/charter companies taking shots accross the bow of each other.
In my experience, mostly Great Lakes based, there are two options available.
1. A security deposit of from $600-$1000 that is fully refundable. Also in my experience a full refund is usually the case. Surprisingly, little abnormal damage is done to most yachts. However, just as in a car rental, if you break it, or total it, you are responsible for fixing it unless you do the following.
2. Add damage "insurance" for a cost of $25-$50 a day. This is non -refundable in all cases. That gives you protection if you do major damage to the boat. For instance, if on the second day of your charter you put the boat on the beach doing $50,000 damage, your liablity as a charterer is the loss of your security deposit, the loss of the balance of your charter time (which you paid for up front), and whatever costs are associated with the time that you must now fill because of your accident.
Not a bad deal if you consider the cost of the yacht compared to the average cost of a rental car and then compare these rates to what Hertz would charge you if you elected to go with their insurance.
"Insurance" above is in quotes because in most cases a insurance policy is not purchased with those dollars. They go into a fund, which can add up over the course of a year, to cover the expenses when and if they do happen.
If you look at the "insurance" as a percentage of the total charter it is silly not to get it. Your risk is lowered and you walk away...sometimes from things caused by your own stupidity.
Again, not sure what our Greek friend is doing, but it sounds like a version of the above. Costs do have to be covered in the end. A !000-2000 Euro deposit might sound like a lot of money to some, but as soon as you have to pull a boat off a reef, replace the keel and rudder, and lose 2 months of revenue, it looks like a paltry sum. You can get insurance for most of the above, however the loss of revenue is not normally covered in a yacht policy.