I tried to reproduce the issue in my test save and could not. The hex room acted just like the square room. Neither let the water in. In your pic, I see the foundations are underwater but, in my testing, my foundations would always be above the water, and I couldn't drag them down below. The game just would not let me do that with MOO on or off. So, I'm inclined to think that you might have a mod that allows you to do that, and that is what is causing the issue. So, are you using any mods like TOOL or something else that lets you avoid the build system constraints?
I also tried with a pool and the game won't let you place a room in a pool so I'm pretty sure you're in Sulani on a beach lot. It's the only place I could place a room in the water.
You said that the issue still happens after removing your mods. Some mods will leave lasting effects even after they have been removed. You might be able to fix the issue by reinstalling your mods and demolishing the hex and square buildings. Then quit and remove the mods again. Restart and go back to the build and rebuild them again to see what happens. I have no idea if that will work but it might be worth a try to save the build.
Hope this helps.
So here’s another picture of the issue, on the same lot in another save, doing the same thing. It’s not in Sulani, it’s in Henford, and it’s a lot that has mostly been turned into a lake. I don’t use building mods, only life mods. The only build things that I use are MOO and alt moving objects.
I replicated the issue this time without any MOO or alt object things. There are no mods currently in my game folders, and haven’t been in for over a week and I’ve loaded/played the game many times since then. And you can place a room underwater when you have pond is deep enough, when you place, it will be under water at the bottom of the lake. Use the arrows you drag it above.
Oh wow! I went back and tested again using a pond. And I get the same thing you get. The square room doesn't get water inside, but the octagon room does. So, I was able to reproduce it. In fact, water shows up in any room that is not square. Octagon, triangle, round, etc. And it also happens when you draw a non-square room with the wall tool as well. MOO on or off does not matter, it happens regardless.
Tbh, I'm not sure if this is a bug or something the devs just didn't anticipate that players would do. Could be either. But I would expect that all the rooms would behave the same way regardless and they do not so definitely worth reporting. Here's a pic from my testing.
Hope this hope..
