I'm Catholic, so my priest suggested we not sprinkle the ashes. There's no specific rule or anything, but burial of ashes is specifically referenced in the Catechism. Of course, Father J. referenced "The Big Lebowski" ashes mishap, not Canon Law, so I was somewhat more accepting of the suggestion. Seemed to come from a more comforting place, I guess. So we're going to bury my son in whatever I can find, just outside our church, next Saturday night.
Of course, in all my infinite strangeness (or as my sisters call it, "strength"), I am seeing a very practical side to this specific situation. Urns cost far too much. Now I've never been concerned with my body after I die. Who honestly cares what happens to their skin and bones and stretch marks (that were totally earned not that I've got the baby to show for it, thank you very much)? I've always said I wanted to be cremated and swept into the dustpan because God forbid someone should feel compelled to visit one location on a regular basis, feeling that this was the only place they could feel close to me. So, here I sit, online-shopping for an urn for my son, still unable to bring myself to spend hundreds on an ugly box that I will see for a total of 5 minutes prior to his burial.
To answer your question, yes, it does have to be water- and air-tight. Otherwise I'd use the gorgeous decanter I have his ashes in now. Once the columbarium is complete, he'll have to be dug up and moved. So for now, I'm in the market for a cheap urn.