If the wood has had time to season, dry out thoroughly, so there is no liquid sap in it, it is safe for sure. Otherwise, it is probably still safe, but I'm not sure of that. It is a very soft wood though, so it will likely always need slate or some other weight attached to it before it will sink. And, soft woods tend not to last as long as hard woods in water. Juniper is naturally resistant to rot, so it may be an exception.