>> like this to work, ideally so successfully that it keeps scaling up.
There are hard scale issues. The wings must remain close to the water for ground effect to be a thing. But the thing still needs to bank to turn. That means one wingtip getting higher to the point it can lose GE lift, and the other lower to the point it might strike a bigger-than-average wave. The longer the wings, the less the craft can bank in a turn. Get too large and in-ground-effect flight becomes impractical/dangerous.
There are hard scale issues. The wings must remain close to the water for ground effect to be a thing. But the thing still needs to bank to turn. That means one wingtip getting higher to the point it can lose GE lift, and the other lower to the point it might strike a bigger-than-average wave. The longer the wings, the less the craft can bank in a turn. Get too large and in-ground-effect flight becomes impractical/dangerous.