Bluenose we did the boat trip in Melissani on Sunday, and two people went into the water from anotger boat. They we're very cold getting out!
It needn't be PADI qualified, which I am, it could be any of the other organisations. It needs some level of diving experience to ensure you don't bump into the stalactities, or, more importantly, stir up the bottom as this would take a long time to clear and so spoil the view for those in boats.
Aquatic World, along with other dives centres on the Island will take non qualified people on their normal dives, but would expectthem to do a "try dive" first. This entails some basic theory, instruction on how to use tge equipment and then a descent to about 5 metres. It's the same procedure for qualified divers who haven't dived for more than 6 months.