The Lake is huge and beautiful. The water is pretty nice but I've never actually swam there. I've been there a couple of times. The first time I was sick and had to stay in my hotel room, the second time we stayed at a very bad hotel. Talking about hotels you can actually find nice hotels on Samosir Island (located in the middle of the Lake), long story short it was a miscommunication that got us to that horrible hotel.
Although every hotel pretty much have the same view:
There are two ways to cross the giant Lake to Samosir Island and back. By car, which would normally require a long time, or by Ferry boat which would take an hour to cross the lake.