Dock framing questions

[Willc]

I have this mess of a dock on a ground water rain fed pond. It is out of the water more than it is in it.The "builder" did not use pressure treated for the deck is why it is in this terrible state. I was going to redo the deck but have noticed the structure is to far gone for that. I would like to just make it a simple gangway type dock using the existing single supports and cantilever 2' off each side of a central beam or should I do just a simple ladder frame ? There is a 4x4 support at the 8' mark and the length of the entire dock will be 16

 

[Willc]

We are currently down about 5' and it fills and floods usually in November for the past 10 years. This is still low but we had water under the dock

 

[Futbalfantic]

There is marine grade lumber. But it is way more expensive.