On my 2000 911CD I had to take a pipe wrench to the seat pole and twist it out. The seat post was tired and it needed to be swapped out so I wasn't too conserned about how the post looked once I pulled it out. Just wrap your seat post with a few layers of Alabama Crome (duct tape) to the seat post for protection and then go at it carefully with the pipewrench.
This is where I am addressing your comment. Once out I swapped in a new seatpost and actually had to make the new post smaller. (I didn't want to open up the mounting hole, because the next seat post that I might need to buy might be made to the proper diameter and actually match the boat). I used an orbital sander on the seat post to make it a bit smaller, (I removed the plastic locking tab so I wouldn't damage it) sanding a little and checking for fit often (you can't add more material). I wasn't that concerned if the seatpost became a little "flatted" from my sanding, just keep working your way around outside of the post. Once the sizing felt still tight but managable, I grabbed a couple of fingers full of lithium grease and smeared the seat-post hole in the boat deck and then worked the seat post in, making sure that it wasn't too tight.
Hope this helps...