My name is***** am a different expert on the forum, your previous expert "opted out."
I disagree with their assessment of your situation for a couple of reasons, and while I am not going to be able to give you a "neat and clean" answer to your issue here, I do believe that I can give you some more nuanced information to help you search out the information you actually need to find out your rights associated with your dock and access to the lake.
A "grandfather" clause exists only if and when it is specifically provided. For example, if a building code requires heaters to be double strapped, but contains a "grandfather clause" so that any heaters installed before the date of the code are exempt - that is a grandfather clause.
Grandfather clauses and rights do not exist in a vacuum or a void.
HOAs (or any form of Common Interest Development - "CID") exist as a matter of reciprocal easements - meaning that your property is either a part of the CID or it is not. You say that the assessments are not mandatory - this means that either you do not have an actual CID or you are not a part of the CID that incorporates your neighbor's homes.
The best way to determine whether or not your home is a member of the CID is to get a complete title history for your home, if you don't have one from when you purchased the property, you can get one for a small fee from any of the national title companies (First American, Old Republic, or Chicago Title). They can also get you copies of the recorded governing documents for the CID.
It is important to determine whether or not you are a member of the CID not only to determine whether or not you have to pay assessments, but also because being a member gives you a right to participate in the governance of the CID - meaning you can vote, and be a member of the board of directors, and review the CID's papers (including past meeting minutes, enforcement actions, accountings, and any other documents).
You then need to identify the ownership of the lake. I take it from the above information that your lake is probably private (but I don't want to assume this). Again, a title company can tell you this for certain if you are unsure. If the lake is in fact owned by the CID, and you are a member, check what the governing documents say about access to the lake (the governing documents can include by-laws, articles of incorporation, CC&Rs, and any number of other documents - sometimes they use slight variations on the name, but the bot***** *****ne is that they must be written down in order to be enforceable).
If the lake belongs to the CID, and you are not a member for whatever reason, then you need to check to see if there is any right of access for neighboring owners - this would be in recorded paperwork filed by the owners with the county recorder's office (once again, a title company will have this information for you).
Given the scope of information and document review that you have to go through, it may make the most sense to hire a property law attorney to help you with this (I have also had customers in GA that have run into problems getting title documents without an attorney, even if they are willing to pay, you can always get this directly by going to the County Recorder's Office, but be aware, this is not as easy as it appears - document research for title documents is a somewhat nuanced task and if you overlook or miss something it is just as damaging to your final analysis as it is critical to finding documents (you might miss something key to your easement rights, etc.), so I do recommend using a professional here).