The 336 seat has a front pivot mount and the back of the seat is coil spring mounted, so you bounce around on bumps. With the boat seat and the high back support it's pretty fun

. There are times I do that little kid sound of "WEEeeEEee" while bouncing up and down
I wouldn't recommend it if you don't have any springs or suspension below. The benefit of mounting your own seat is you can choose where the pivot ends up, which makes a big difference.
EDIT: Wow, nevermind! It appears I must have either A: had too much money when I bought these seats or B: found an amazing sale, because
these boat seats now cost $200! They are comfy, though.