When I went to buy mine, the salesman specifically noted that the seat was narrower than the traditional Sea Doo seat. Boy, he wasn't kidding! I think the idea is to have it be more like a motorcycle.
It's great for standing up and jumping waves and probably more comfortable for smaller framed riders, but not so much for a long trip. But then, this isn't really designed to be a cruiser.
The other day I was running across the lake about 5 miles, and coming back I folded a towel and sat on it - partly to widen the seat and partly to keep the towel from blowing away. That helped. It probably wouldn't be hard to make a pad that fit over the seat with a strap going underneath to add some width.
Maybe something like this?
I have the 2up ... is the seat wider or more substantial on the 3up?