The Bitter End
One would think that this is a term which came from ashore and moved into ships. However, how the term "bitter end" actually moved the other way.
In modern ships the "bitter end" is the end of the anchor cable (chain) which is secured to the bulkhead inside the cable or chain locker. In short it's the final link of the anchor cable.
It came to be known as the "bitter end" back in the days of sail when the anchor cable was made of rope. The securing points on ships and piers may have looked the same, but they had different names. On the wharf, wall, pier or jetty they are known as bollards; in a ship the same device is known as a "bit". It is simply a place where a rope or line is secured and which will hold fast, usually two posts or pins.
In older days when the anchor was secured to the ship, the end of the anchor cable was secured to the ship's forward "bits". It was then led to a windlass where several turns were taken and then out to a hawse hole where the other end finds the anchor. The end of the cable, furthest from the anchor, secured to the ship's bits, was known as the "bitter end".
Trivia: There is a story around some navies about "The North Pacific Deckpecker", which apparently pecks away at the nonskid paint on ships' upper decks, particularly around gun mountings. How old is this naval myth?