poly is a strong compound esspecially marine grade used on boat decking this is were i got the idea from, using it on my boat. I 've never seen it wear in water, unless something roots or eats or constant abrasion. I'd imagine you would notice unusally wear and inspect it for any problems, but applied at 3-5 coats it is very thick and strong look at any dinner table finish that glossy topper is polyurethene, and it looks deep and that is only two coats typically. Yes it will eventually break down but by that time you'd get your moneys worth out of it, and probly be board of that particullar drift wood specimen. Maybe in a marine setting it dose wear faster, but 5 years on a boat floor it took to wear in spots. I haven't seen any chemicals leech from my drift wood in the 3 monthes its been in my aquarium. I watch my water parameters daily and nothing out of the ordinary has ever been found. If polyurethene bothers you can buy liqiud acrylic in cans and you can typically find it in wood finishing shops/stores and home good centers. This stuff is easy to work with but us a wee bit mess make sure you wear clothes that you don't mind destroying and lay down drop clothes. Traditional ways are time test and work, but for the impacient old fashion take too long and gives way to new ideas.