I have the same problem - and it is the graphics card - and there is no fix as of yet....
I have an ATI 2600 HD PRO 512MB card... and it can't touch the high seas. Lucky for me I have a second card (a junky built-in Intel 945) which does work, and it allows me to sail - BUT - it upsets me to no end that I have a high performance card and can not use it...
Tried updating the drivers, uninstalling the game and reinstalling, even formatted the computer and just put the card and pirates on it - and no luck.
Sorry to say, but you may be stuck... best of luck.