Without reviewing the documents, it is difficult to give a complete response. However, based on what you tell me, and based on my experience as an attorney who has represented community associations and those in disputes with associations, I can give you general guidance.
As to question #1, I would imagine the owner is permitted to withdraw from the dock owner's association without selling the home and/or moving out. I have seen situations where there are extra amenities in a community which owners can choose to be, or not be, a part of.
But as to question #2, there are certainly consequences to the decision to withdraw from the dock owner's association, which would include a loss of any benefits that come with this membership. Here it would seem those privileges would include use of the road, ramp, and dock. Also, from what you tell me, it sounds like the community documents are clear in that dues must be paid in order to access the docks. In situations like this, generally a letter would be issued warning that privileges will be revoked on a certain date of this situation is not rectified.
I hope this is helpful. If so, please provide a rating so I receive credit for responding to you. Also please feel free to ask follow up questions, or upload any community documents it you would like me to review them. Thanks!