There's something strange in the Seto Inland Sea

I bought a new phone last week. For those who care, it's a Nothing Phone 3. While getting familiarized with it and its camera last Thursday after work, I realized that I hadn't taken any photos of the Seto Inland Sea for a while (I usually post these on Mastodon or Pixelfed). So I went to my usual spot and... there was something strange.

Three platforms on stilts, a couple of hundred meters from the shore.

But what could they be? In my 17 years of visiting the Seto Inland Sea and my nine years of living almost on the coast, I have never seen such things.

They don't look industrial, nor are they for fishing or an art project (Especially not at this spot — maybe if it were closer to Takamatsu Sunport or an island, but not here).

I tried to zoom in, but as I discovered a few days before, the zoom is clearly the weakest part of my new camera. It shoots at 50MP by default, but switches to 12MP when I zoom. Shouldn't it be the other way around?

There are a bunch of things on top of it, but I can't really tell what it is.

Right now, I have three hypotheses.

It could have to do with laying cables in the sea, which could make sense because I had seen a 'cable-laying barge' the day before. It's a very large barge with an impressive crane on it, and if you're lucky, you can sometimes spot it in the sea. However, as I mentioned, installing or replacing cables under the sea is not a rare occurrence, and they have never put such things before.
It's located off the coast from one of the small marinas in town. Are they planning to enlarge it? I'm not sure why they would do that. I doubt the number of private boats is increasing.
Are they planning to build something bigger? An industrial port, perhaps? Or new reclaimed land? I really hope not. It would ruin the landscape. Sadly, I already know that the local authorities don't care about such things.

The platforms were still there yesterday. I haven't been to the coast today. I might go later, closer to sunset, though: today is another hot day, just like pretty much every day since early July. Right now, it's 36°C and feels like 42°C at 4 p.m. – a "normal" summer day in Takamatsu these days!

If you have the slightest idea what it could be, please let me know in the comments. I shall try to find out more.