It's only 24 miles wide at its narrowest point. You're right that they couldn't clog it completely, but making the waters un-navigable for large oil tankers seems doable. Especially since the crews wouldn't just have to watch out for the sunken ships, but also large and dangerous chunks of floating debris and hazards they can't see under the water.

The imagery conjured by "death valley" to me is a boat graveyard - endless wrecks from shore to shore.