If it's been in salt water it's not a good idea, since old wood is very porous it would probably continue to leach salt and other minerals even after it's boiled. If you want use found wood, you should get some from hardwood species, like oak. Definitely avoid softwoods and especially evergreens, like cedars--they have toxins in their sap that help the tree kill insects. Cedar really shouldn't be kept with any animal for this reason, even though many companies market cedar chips for small mammals, it really isn't good for them at all.
Once you find a hardwood piece, you have to clean it very thoroughly, boil it, and soak it. You can find more detailed directions on how to do this by searching a little bit. It's really easiest just to buy pre-soaked wood from Petco, though.