We should attack Saddam NOW because he MIGHT develop nukes and then he MIGHT develop ICBM's that can hit us.

By that same logic, I should start shooting anyone in the city who MIGHT buy a gun and threaten me.

Another difference between cargo ships and ICBM's is that the cargo ship approach would be a terrorist act (unless we had attacked first) while the ICBM approach would be the traditional MAD approach.