Most wood will rot in time but chances are that if you found it in a river already it will be safe to use. That said, to air on the side of caution I would use it on a tank with less expensive inhabitants first if it were me. But wood disintegrating under water is also enjoyed by a number of plecs assuming it is not of a safe variety. Hard to tell which tree it's from if it was already in the river.
Most of my wood is found this way and many of my much larger base pieces 20-30 kg have disintegrated to 10-15kg over 5-6 yrs.I
the piece you have actually looks burnt, that might be more of an issue?
Will take a couple of weeks to absorb water and sink again maybe a little more, or rather to stop being bouyant.
Certainly an interesting piece though.
So far tiny pieces came off, no tannins, will probably find something safer as I want to be safe if it was burnt. Might used this as decoration for my frog or geckos.