Isla Victoria is the largest island in Lago Nahuel Huapi. You can visit on your own but it is much easier with a tour group.

This guided tour on a catamaran visits both Isla Victoria and the Parque Nacional de los Arrayanes. You can follow along with the group or go on your own wander as long as your back in time to board. I would expect a large tour group and maybe no English speaking guide. Many advertise English speakers and then don't comply (according to many reviews). I've been on this tour and enjoyed it but it is a large group.

If you want a more personal experience, I recommend a private boat excursion, I went with these guys on a later trip and it was spectacular.