Hey there @Son_Akira! I actually ended up noticing this as well. The water system came with a custom collision and a special post process volume, so I would have expected that the water volume would have been as well, but upon closer inspection it doesn’t look like it has been integrated! So there’s a number of ways to deal with this.
One would be manually placing the physics volumes, which as you noted could be tedious if you have lots of water.
Second, you could write a BP that does the math between all the splines (for lakes at least) and takes the depth and makes a custom physics box for it automatically. This would be tedious at first, and prone to error for special volumes, but could cut down dev time if you do it a ton.
Third and my favorite: Create a custom movement style and just use the collision that comes with the water system by default. This way you don’t need the physics volumes and can customize how it handles in case you need some special water movement. Best thing about it is if you set it up well, it’ll just work right out of the box! I made a little version of it really quickly just to show what I mean.
Hope this helps!