See pond sliders#20 of 30 things to do on Crete
You can reach the habitat of these turtles in a boat, a catamaran or kayak. As a rule, many people want to look at the turtles, so, if you dont want to get lost in the crowd, go to the eastern part of the Kournas lake - there are a lot of turtles here, and few people.
They swim in large groups, and they are not afraid of people. When they come close to the boat, you can touch them, and if you meet friendly turtle, you can even hold it in your arms :)