You can't restrict access to waterways that other countries further upstream depend on to function. If this were the US cutting off access to the sea for say, the Gaza strip (which they already help Israel do, actually, so I am definitely not saying they are morally right here), then you'd have a much different opinion on it, wouldn't you?
Almost the entire world recognizes this basic courtesy of the sea. It's like damming a river for yourself and starving out others who weren't as fortunate to live closer to the source.
Either we have free access to the oceans and waterways for all, or we just shut down international trade and all become hermit kingdoms.