rigidbody.AddForce doesn´t work in c#

Hi, I´m trying to make a FPSCharacter swimm in water at a specific height level. I´m using rigidbody.AddForce to add a force that pushes the rigidbody up. However, AddForce isn´t working. I saw annother Question and it seems like i have to unchek "is kinematik" for it to work. But if i do that, i can run through the terrain and i get a few other Bugs too. If you know how to use AddForce with a kinematik rigid body, please let me know. And also, if you know another way to let a rigidbody swim on water, please let me know to.