How come ground-effect seaplanes never became popular for military or civilian use?
I would imagine the increased fuel efficiency would be a huge draw for both, and the maintenance and training would probably be easier than aircraft too since there isn't as fine a line between "can fly" and "can't fly" for something that doesn't need nearly as much power to fly.

Personally, I'd love it if seaplanes like this replaced some airlines, since they'd probably be a lot cheaper to fly on. Sure, it might take several days for an international trip, but it's worth it if it saves a couple thousand dollars.