I used to have an extremely unusual and beautiful piece of driftwood that was a hand-me-down from my mother, and it was a gulf beach find. Ocean-formed driftwood is often the most beautiful due to the harsh conditions that cause the erosion in the water, and then being tumbled by waves with sand, stones and seashells. Mine was a continuous, unbroken trapezoidal ring with two main branches extending at a 45 degree angle to each other, the center being eroded completely through. It fit perfectly framed in a 29 gallon tank and looked awesome with plants growing through it. I'd still have it if my ex-wife hadn't burned my house down.
As for safely using it in your aquarium, since it came from a marine environment, there's less chance of contamination by biological pests, because most of anything that was living on it in the ocean won't survive in a freshwater environment. That said, and depending upon the location where you found it, there is the possibility of chemical pollutants soaked into the wood, especially if it was in or near a high-traffic area (like a port) or an industrial area, or near the mouth of a river that is industrialized. Boiling (if small enough) or, if large, soaking in tap water for a few weeks should reveal any unwanted pests that may happen to develop, and should allow the majority of any toxins to leech out, especially if you change the water out every few days. After that, throw in enough bleach that it smells like an over-chlorinated public swimming pool, and let it soak some more. (Be careful of using too much bleach or you will end up with bone-white wood like I did once. It looks ok, but it's not what I intended.) Soaking in bleach water shouldn't be for very long if you don't want to actually bleach it; a few hours should kill most anything. After that, consider it sterile, but needing to be dechlorinated, which you can do either with your standard aquarium dechlorinator or again soaking it in tap water to leech out the excess chlorine, either way until you no longer smell the chlorine. You could also air dry it, but that would make it more buoyant again. As for tannins leeching into the aquarium after all that, if you don't like the look (some people do, some don't), they can be removed by filtering through activated carbon. (Some fish--especially rainforest species--actually prefer tannin-rich water.)
These would be "textbook" procedures, to be absolutely sure of no contamination, but most of us cut corners a bit if the wood and where it came from look clean enough and there are few places (cracks, holes, etc.) where something can hide. As mentioned before, get as much soft wood off as possible, and do this before you decontaminate it--no use wasting effort and/or chlorine bleach on soft wood you're going to get rid of. I do that with a wire brush, either hand-held or the drill mounted type; the drill mounted type works very quickly, but is too rough to use on thinner, more delicate pieces. I have used the drill on a piece I thought was fairly solid, but ended up with not much more than a few toothpicks! The drill/brush will eat up soft wood like a stray dog on a pack of hamburger meat! A mechanic's steel brush or a BBQ grill brush work well.
Good luck with it, I know you'll enjoy that piece so much more than one that could you buy, because it is something you discovered for yourself. I always enjoy the decor, fish and plants that I find that much more, because they bring back the memory and feeling of adventure that comes with the search and the thrill of discovery from when you find it.
"May the Fish be with you."