We cruised the "Mariner of the Seas" recently, and they had a great jazz group in one of the lounges. Not meny people knew they were there, so it wasn't very crowded. It's a shame, as they were really, really good.
We just returned from the "Carnival Glory", and they too had an excellent jazz group. I agree that most of the larger ships are going to have a jazz combo or group aboard.
Good luck, and see ya aboard,