I just wend for a nice trip to Karimunjawa (2 weeks ago). It is a nice place to be if you like the beach, swimming, snorkeling, diving and if you dont have problems with being on a boot.
It was awesome and during snorkeling amazing pictures are made. You can travel to Karimunjawa on backpackers budget but also more luxury.
The sentence; with a little tourist, unfortunately it is not true. If you go during the weekend it is busy and it is becoming more popular for tourists. But during the week you wont see many.