OK This is the simple version since I have to go to a inspection this morning.
1.You have to get in the water PERIOD, no getting around it.
You have to see how much rot and decay is subsurface and more importantly the contact points with the substrate material. You will need a 18 inch long to 30 inch long Probe (screw driver works great Yes they make them that long) and it needs to be thin. You check the first contact area with mud/sand ect and all around the top edge as the pillar enters the mud/sand. You will see the most damage here. (below the mud line there is far less oxygen and less rot BUT it does happen so use the long probe at a angle and see if it penetrates the pillar.
Make a note of the depth in the mud of the pillars and what type of material they are in mud / sand ect…also as you get further out on a dock the material can change from sand to mud. This info will be needed for evaluation of the SWAY you may have up top further out.
As a general rule width of dock will determined pillar depth needed for stability but it also depends on what the pillars are buried in. Sand is much stronger than mud ect… So nothing is hard and fast on exact depth.
5 How where the pillars planted? Piled driven in or water jet plunged? Why Pile driven can crack a pillar at the base and the builders never see it.
Surface wood gets sun bleached and cracked so its replacement is much sooner than good quality pillars every time. general rule is 2 to three skins for ever re-pillar replacement (2 skins being the most likely here in SC)
Pillars size and quality is real important on dock longevity. Big pillars make for a long lasting dock, and will reduce the ownership cost by thousands.
Construction framing is all over the place but generally a good dock has good lateral support…not just poles in the ground and some 2x6’s as rails and skinned.
Best skin material by a mile is TREX or one of the composite new deck material’s, no splinters and weathers GREAT but dang expensive. (2x the cost of good treated lumber but last 2 times to 3 times as long)
The big one—everyone Try’s to put electricity on there dock and most of its is DANGEROUS. So look carefully at how it was done and what would happen should the water level rise?
I have 128 foot dock with a 30x50 platform on the end. Cost to replace all of it? estimated at 50K Cost to re-skin (new deck wood and rails) 5-7K Treated 2x8 wood. My Pillars are 8X8 some as long as 16 feet. ( I have owned 6 lake homes and one ocean home, and grew up on a lake) Been fixing docks for years.
Bottom line there is allot more under water than on top, any NACHI member could evaluate the wood and general construction but its under water where you learn the real truth of a docks condition. Every lake home I ever bought I dove the pier and checked it…bad pillars mean new dock is in your future.
Beware of the new skin on old pillars. You will see lots of cheapo pillars also like 4 inch pine garden poles…worthless and dangerous in a year of so.
This was not real technical it was just to show you some areas and how to look generally at a dock.
If your not PADI certified your gonna be holding your breath a long time! Oh you can get the bends in 3 feet of water on repeated dives so don’t think a mask and snorkel and I am ready…take a dive class and know the dangers of multiple short breath dives. Also ask your state folks what the regulations are for diving for pay.
Hope this helped,