The Pletna is a sort of gondola, steered and motored by a muscular chappie with 2 oars. There were none on the lake the day I was there, but as you can see from the photo, the Pletnas themselves are very photogenic, and I do have a soft spot for traditional wooden boats.
The general programme is that you pay the ferry man for the half-hour or so trip to the island, he leaves you there for another half an hour and brings you back. I can't speak from experience however.
You can hire you man of choice from a number of points round the lake (see transport tips) and just take off. I can't guarantee a serenade, but who needs "O sole mio" when you have such an amazing view?!