Hard to tell with wood you pickup, it could contain all sorts of nasties. Perhaps shove it in a bucket, pour in boilding water and leave it to soak for a month, with the odd water change here and there. not really sure what else to suggest.
Boil it a few times if you have something it'll fit in, or do as Tizer says and pour boiling water over it. It'll need a good deal of soaking/water changing as I imagine it will have soaked up a fair amount of salt.
As above. Personally I wouldn't use wood from a beach as god knows what is already soaked into it. I think some bits from lakes is good and have used these.