I haven't used polyurethane, but I have done a couple paludariums using epoxy.
At least one with some epoxy bought by the gallon from a plastic supply place, and a couple smaller ones just using the stuff you can find in craft stores (scultping/mold stuff), or in hardware stores (paint/varnish stuff). It's typically advertised for making those bar/counter/table tops that have newspaper clippings, photos, and trinkets embedded in them. Usually advertised as being 'equivalent to 50 coats of varnish'. Not sure on the exact brand, but I've used a couple different ones and had decent results.