Arguably, the most destructive marine borer is the Teredinid (also known as Ship worm).  Evidence of Ship worm is sometimes not apparent until structural failure occurs.  The worm is a filter feeder that can also consume wood.  It enters submerged timber piles through a pinhole-sized opening, using the pile as a safe home while it grows.  Terednids are reported to grow in excess of 2m long and have cylindrical burrows usually around 10mm in diameter, but as large 25mm diameter.  Surprisingly the largest and most destructive variant is found in brackish water, however, they are generally most active in highly saline water (i.e. on the coast). 

Ship worm activity is increased in clean water, meaning that environmentally responsible practices contribute to an increase in the decay rate of submerged timber.  This is a small price to pay for cleaner waterways.