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.
From the research I did about getting your own wood you do not want to use soft woods. Use only the harder woods. Once you find one you like, boil it for several hours then let it soak for several days-weeks. I live in NC and there isn't really anything here I liked from walking around the local ponds, just a bunch of rotting soft stuff. So I bought a piece, this website has the best quality and price compared to most of the other sites I came across on online. Once you get it you still want to soak it for 1-2 weeks to leech out the tannins or your water will turn yellowish.....unless you want that, I believe it will soften your water and some people going for pure amazon plants and look like the tannin water.