Having Immutability by Default in C-Sharp and Java?

F-sharp is a functional programming language, but it's not the equivalent to any other one. In C-sharp and Java, you can make things immutable by default; in F-sharp they're exactly opposite. I would write code in a way that other people could maintain if I was working on an F-sharp project. It's like fish don't know about water, right? You're not even aware of it by the way.