Despite the Hollywood swashbucklers the sword probably ranks as the third most important weapon in the pirare arsenal. The sword was a weapon of great psycholgical intimidation. An trained swordsman had an enormous advantage over a novice. Swords took a certain amount of skill to use effectively and the unskilled swordsman knew he stood little chance to a skiiled swordsman. Often a show of skill was enough to make an unskilled fighter turn and run. In order to become effective with a sword, one had to be taught swordmanship and then practice it for several hours a day for years on end. A true swordsman would probably practice daily.
Despite all of the practice, the swordsman at sea would have to adapt his skill to a new fighting environment. A Swordsman on the seas had to learn to fight in a close, confined space while being mindful of several other sword fights occuring all around him. In order to make the most of his knowledge he would need to control the situation and force his opponent to fight his battle.
Typically the sword fighters were not paired off into a duel. It was more of a mad dash to a point, hacking at who ever blocked the path, while trying to avoid being hacked. Both the pirate and the merchant would prefer to stab or hack a man in the back that facing him head on. This meant that the pirates would not be fighting duels but fighting in teams of two or three, trying to get advantage on a weaker foe. The merchants would also try to do the same thing or draw the pirates into tighter spaces where they would have to fight face to face. When this happened, the ability of the swordsman would come into play and the man who was a novice with a blade was at a grave disadvantage.
To further complicate the sword fight, the swordsman had to be ready to fight against, pole-arms, boarding axe, etc. The chaos of a battle aboard ship was everchanging and unpredictable.