How I check waterside posts for rot before I trust anything near the beach

Same experiance on my end, but on a small lake dock rather than salt water. I learned the poke test the hard way after one of my corner posts snapped during a windy fall. The top twelve inches looked fine and had decent stain on them, but six inches below the waterline the wood was mush. I coud push my thumb into it without the screwdriver even being needed at that point. One thing that helped me after replacing that post: I wrapped the new one in a plastic sleeve from a roofing supply place before setting it, leaving the top open so water can drain out. Three years in and it still feels solid when I poke it every spring. My neighbor just slathered his in roofing tar and buried it, and his posts went soft way faster.

I also started keeping a cheap moisture meter in the dock box. Not fancy, one of the thirty dollar ones from the hardware store. Readings above twenty percent in a post that's been out of the water for a day always turned out to mean trouble somewhere underneath.

The part that surprised me most was the hardware. My lag bolts looked perfect from the deck but the wood around two of them had rotted from water running down the grain into the holes. Now I caulk around every fastener head in the fall before freeze up.

Salt water is a whole diffrent animal for the hardware too, not just the posts. I ran a boat slip for a decade and the hot dipped galvanized bolts I used back in the day would still rust through in five, six years near the splash zone. The old-timer who taught me swore by pulling a couple of test bolts every spring and looking at the shank, not the head. If the threads are eaten down you're on borrowed time. These days I'd use stainless anywhere salt spray reaches, even though it costs more up front. Cheaper than a deck coming down with someone standing on it.

The part about salt water eating fasteners rings true. I keep a boat at a tidal slip and swapped every bolt on my gangway for stainless after the galvanized ones let go in under four years. One thing to watch in salt water though is what's inside the wood, not just the surface. Had a piling that passed the poke test above the waterline but the crab sure had riddled it with little holes right at the tide zone. Looked solid outside, hollow like a straw inside. Now I tap each post with a mallet every spring and listen for that empty drum sound.