The Saudis have a tidy income. Let them build or hire their own deterrent.
IF the Saudis don’t like what the Iranians are doing, there are plenty of oil terminals and pipelines for the Saudis to retaliate against.
The problem would be that if Iran or SA decided to start sinking oil tankers in the strait, NO ships could get through-in a re-run of the Iran-Iraq War of the early 80’s. This would cause oil prices to spike because, like it or not, the worlds economy runs on oil and these countries are some of the biggest producers. If, say, Japan cannot get oil, they have no money to buy our, or anyone else’s goods, which will cause a worldwide downturn in economies. I’d like nothing better than to see them both ruin each other, but I don’t believe in cutting off my own nose to spite my face.