How NavMesh Cost can slow down NavMeshAgent's speed?

Hi I have a terrain mesh and a water mesh in my scene. I make them in the different layer for Navigation. Terrain mesh in the "default" layer and water mesh in the "water" layer. I set the "water" layer's cost to 100. What I want is the "water" layer still can walk through but it should make NavMeshAgent walk slowly if they have no choice need to go through the "water" area. But I dont know how to implement this. How can I get the NavMeshAgent's layer cost for current position? How to determine the NavMeshAgent already in the "water" area?