How do I Spawn an Object on a Seafloor Randomly?

So I am using Unity 5.5.0b2 and am not adjusted to using the new Physics updates it has. I want to spawn an object randomly using Random.Range along the seafloor (seafloor has random curves) and have it align itself with the slope of the seafloor that it lands on. It does this all in the OnBecameInvisible function. Then, when it becomes visible, I want it to not move anymore (either on its own or when impacted by collisions).

In past versions of Unity, I would just set the object's Rigidbody2D to kinematic and that was that. But now setting it to kinematic does nothing.

I'm just looking for the best way to go about this. I am using UnityScript.

Any help would be greatly appreciated! :D