Cloud seeding alters precipitation patterns by single digit percentages. While this might be economically important, it would probably not be an effective method of hydrological warfare. Only a very select set of places could be targeted, it would take a very long time to have any practical effect, and it would be obvious long beforehand what was happening and who was doing it. Seems like a really easy way to give your neighbor a casus belli against you while in no way restricting their ability to wage war.