I have searched all over for Serpents hold. Walkthroughs say it is south of Buc Den but where? Please help.

Chastain Dragon

It has been so long since I tormented myself playing Ultima 9, to the point remember how easy it to move between places separated by water, but it should be in the same place as other games.

http://gigi.nullneuron.net/ultima/u9/maps/world2.png

The Ultima 9 world map, it should be on the largest of the 3 small islands on the center of the southern edge of the map.

I'm rather late in this, but in case it helps either the original poster or another:

If I'm not much mistaken, in Ultima IX Serpent's Hold is sunken beneath the water. To find it, go to the islands in the area and look for a submerged entrance. (Breath potions may help--although I forget whether they're required.)