I've been to both Cumberland and Dale Hollow. I highly recommend you go to Dale Hollow instead. Its just a bit further of a drive, and a much nicer lake. A lot less commercialized, and it has a lot more beaches, rather than Cumberland having a lot more cliffs.
If you decide to go there, we rented a boat for 2 days from Cedar Hill Resort and was very happy with it. The owner was very friendly and the boat was in good shape.
We'd also rented one from Dale Hollow Marina. Don't make that mistake. Dude called me the day before the rental and said "the bilge pump is out on both my pontoons, so I have to cancel, I'll call you if we get anything fixed". Never called....and there isn't a bilge pump on a pontoon, so he was BS'ing me.