A harpoon in particular I don't see being much of a problem unless half a dozen or more were to make it through and get good hits. Exocet basically the same.
A harpoon would certainly sink a modern ship (which is what it was designed for) easily enough. Iowa class armor was designed to resist kinetic penetrators. Its the difference between a 2000 lb supersonic explosive armor-piercing warhead and a 400 pound subsonic missile warhead. One is putting something like 6 times as much energy into the target as the other.
The way to hit an Iowa is from above. Trying to sink it at the waterline is not going to work very well. And remember...its returning fire the entire time.
The way this sloped armor is arranged, its not just about punching a hole with a shaped charge. There are layers and voids.
You can sink anything if you keep pummeling it. But this wouldn't be the same as one-shotting a destroyer or cruiser. You'd have to get quite a few good hits. Hell, you blow the entire bow completely off of an Iowa class ship and the damn thing still wouldn't sink. It would just go slower.