I always travel with an emergency ration of drinking water in a clear plastic container.

I never had a problem with this in North America.

I twice had a problem here in Europe, once at GCI (Guernsey), once at HAM. On both occasions the screeners claimed that unmarked containers were "not allowed", on both occasions they let me keep my water ration, "just this once". On both occasions I subsequently sent written complaints, and had letters back confirming that the screeners had been wrong (I now carry these letters with me whenever I travel by air). These two incidents occurred in 2007, I haven't had any issues since.