I concur with Mac, that Will was able to fulfill his duty (at least to a great degree), but disagree with the conclusion that the barnacles directly correlate with the dereliction of said duty.
Now I will bring up two points to clarify my explanation.
The Curse of the Flying Dutchman
In short, the Curse of the captain is this: You are to ferry the dead lost at sea for all eternity. One day at land for every 10 years at sea. You lose your heart (physically) to become "one with the crew, one with the ship". In other words, you are bound to the crew and ship by your life. This is the curse- you are unable to leave. You can only be "forcibly removed" from your post as captain by one who stabs your heart.
The barnacles can only be explained as a natural occurrence of underwater inhabitancy. All members of the crew (as well as the ship itself) collect barnacles. I wouldn't say it is a direct influence of the curse, as a direct consequence of dereliction of duty leads to the eventual loss of what makes you human. So Davy Jones became ol' squidface specifically because he chose to take a few centuries of vacation time.
Will has barnacles because of extended time in the ocean. Not because he didn't ferry around the dead people.