Flow Simulation does not have the capability to simulate free surfaces (i.e. moving fluid-fluid boundaries.)
Well, my problem is a bit different: there is environment air (as a free surface) and moving water down. I tried to set "Environment pressure" as a Boundary conditions but I think FS interpretates that as an outlet, isn't it?
Are there no tricks to make it possible?
Thanks in advance.
You can have air and water in the same simulation, but only as separated sub-domains.
The "environment pressure" boundary condition simply means that whatever fluid touches that surface will be at the environmental pressure set in the project options. I recommend going through a few tutorials on the software to get an idea of the basic types of boundary conditions.