boat dock repair

[BrookTrout]

I am planning on redoing the decking on my small boat dock on Seneca.  Area doesn't freeze.  Dock is for lift access only, about 30ft long, 4-5ft deep at end.  There are currently 5 pairs of metal tube "pilings".   I assume they are driven into the bottom.  Area is sand/cobbles.  Dock seems to wobble a bit but might be due to loose connections and rotten wood.  My question is how deep should pilings be driven.  One needs to be replaced and maybe more depending on wear once I get it taken apart.  I was also just curious.  Couldn't find anything too definite when I searched it online.  Lots of stuff about seasonal docks...   

 

The pic below shows the dock