You would expect that the longer rod would help with distance as you are using a longer lever to move the tip over a longer distance. I find the difference between an 9' and 9'6" rod to be insignificant most of the time when fishing from a boat. I have a 9'6" 9wt rod that I use in the ocean sometimes. I won this rod in a raffle and wouldn't have purchased it, but since I won it I use it. I find no functional difference between the 9'6' and 9' rods when fishing from a boat. I find the effects of casting from a rocking boat to be more significant and variable from cast to cast than the benefit of an extra 6" of fly rod. Even in calm water I do not notice a difference.

I do find using a longer 11' switch rod to provide noticeable distance improvement if the boat isn't rocking as much, expecially with two handed overhead casts.