For protection from sand and moisture you could take a look at the Leisure Jacket from mEdge:
http://www.medgestore.com/products/prs900-leisure.psp
Just as Worldwalker suggested a BookArmor for general protection. I have one for my PRS600 and I'm extremely happy with it.
http://www.extremeinkjet.com/BookArm...okarmor900.htm