It occurred to me that the increased efficiency might find these a niche as rapid trans-oceanic transport, but 15 feet from the surface is too low for waves likely to be encountered in ocean storms.
Planes colliding is very rare but it does happen occasionally even though they have 3 dimensions to work with. If you'd compress that traffic to two dimensions and then add slow moving commercial vessels, sail boats and weather into the mix I fear it won't end well.
At some considerable increase in fuel consumption Ekranoplans can 'jump' up a bit, keep it up long enough and you won't have enough fuel to reach the destination. This is also how they make turns. There are also hybrids that can fly at some altitude like a normal plane that just exploit the ground effect to increase efficiency. As opposed to those that are almost always in ground effect.
These are super interesting craft, you can easily spend a week just reading about them.
So the answer would be to provide controlled areas for the ekranoplans to travel and monitor those "seaspaces" for infringing traffic.
They're also rare in absolute terms, even including the near-airport ones.
I’ll stop hallucinating now.