I know the OP is old, but I thought this may be of interest to everyone who has spent endless hours at the helm.
When I was a younger man I spent a huge amount of time at the helm of several fishing boats. Many were in the 18 to 25-foot range, all were fast, and nearly all were equipped with comfortable, stable, helm seats. Even when bouncing off waves, the helmsman usually had comfortable seating.
Most powerboats have helm-seats that are contour-fit with padded arm and back rests and thick foam cushions to sit upon. The best ones are not cheap, but once you've installed one and used it you would never go back to sitting on a conventional sailboat seat--there's a world of difference.
I purchased and installed the Cape Cod helm-seat from West Marine last spring. They had a special sale going at the time and I saved about $100 over the regular price.
The seat is high enough for me to see over the cabin top, it swivels easily to any angle, has positive locking, and every person that has used it while sailing with me said it makes a huge difference. In many instances I had a passenger take the wheel while I went below to make a sandwich and when I returned they often said "Relax and eat your sandwich, then make me one and mix up a Margaretta to go with it." After complying with their wishes, they continued to remain in the seat.
The only thing I may change would be the pedestal. The one that came with the seat leaves the post in place when the seat is removed. I will likely switch this to one where the post can be removed from the base, leaving just the base unit in place. Just makes it easier to get around when scrubbing down the cockpit.
You can find the full specifications at TODD Cushion at West Marine