As any sailor will tell you, it is seldom as smooth as the water that you always see in photos of these birds under flight.
It's like tourism photos of Ireland, where green fields are always under clear skies.
Think of these vehicles as floating on a layer of "thick air" above the ocean. They bounce through that air like a boat on the waves. If the layer is rough enough they will bounce more deeply into it the same way a boat will sink the bow deeper into large waves. If the sea is rough enough they go through the ground effect layer and hit the wave below. Bouncing off the waves without crashing necessitates a reduction in speed that kind of makes these vehicles useless compared to boats.
The problem is that just one monster wave is disaster. The bigger you make the device, the bigger the wave it can take. But to justify a bigger machine, you need to have it transporting a higher volume of people. Which means that you only hit the break-even point for ground effect vehicles where lots of people want to quickly get from point A to point B over relatively calm seas. And ideally a relatively short distance so you can pick up lots of fares for your time spent over the water.
However wherever you have those conditions, you have the perfect economic conditions to build a bridge or a tunnel. That may be more expensive, but solve the problem better.
They do have limited military use. There you only want to use it once, the people you are transporting can accept higher risk than civilians would, and being able to ignore mines is a huge win. But again, only some military theaters benefit, and getting the machine where it needs to be can be a challenge.
A more reasonable approach might be hovercraft ferries, like they have in Dover, UK. At least the equipment already exists.
A big issue they all share, with all other seaplanes, is that taking off in rough seas is terrible, and very stressful on both passengers and superstructures.