If it's a seasonal dock, I think you can have a 6' x 40' dock without special permission. Seasonal docks do not need an advanced permit I believe, only what's called a Permit by Notification which means you can st it all up and then notify the State after the fact. Not sure if you still need to get a building permit from your local town prior though.
The following site is useful in determining this type of thing. I've also emailed DES and gotten quick responses before.
http://www.des.state.nh.us/wetlands/PBN/