This musician had a friend build the tank for him. It was driven out onto the Fest grounds filled with water (before all the mega stages came into existence) on a big flatbed truck. I'm dating myself here. Next thing I notice is this fur-covered van bouncing along the infield toward the stage. He gets out, goes up on stage, plays a few songs and then climbs up a ladder and into the tank on the flatbed. He puts on a diving helmet, submerges and holds his hands out of the water. Another guy sits on top of,the ladder, holds the electric keyboard above the water and the musician starts playing. Only lasted one song. Some years later when I had an opportunity to tell him that he provided one of my most memorable Jazz Fest performances, he told me that he made the diving helmet himself and he almost drowned. We had a good laugh about his performance. 😄😃😀
And, the answer is:
Ironing Board Sam