Once, in Penang. All the signs said you must use the machines but most people couldn't figure it out. I managed to get a baggage tag - success! Then...
Actually, it's ideal. All the boat's motions require a balancing response. Your core muscles will be in tip top condition.
The balls are a bit heavy...
Yes, after 30 years of back trouble I've tried most things. Mine was a little less fancy than your picture without any back support. I am fairly tall...