anything you like..if its near the water and has been in it...or out i guess I have done both
look for anything that is solid...try to push your finger nail in to the wood..if it pushes in it is rotting..no good...nothing flaky ..bark can be removed so no worries there...basically anything that you cant scape chunks off of with your finger nail or push your nail in to should be fine.
the heavier wood the better from what i have found in my collecting ^_^
Oh wow. I didn't know it was this easy. But it can't be from a tree, right? Has to be dead? I can always give it a once over with a palm sander then powerwash it.