How to serialize NavMeshPath to sync across network?

You could turn the Vector3 array into a list und use the SyncListStruct class to create the network syncable container:
https://docs.unity3d.com/ScriptReference/Networking.SyncListStruct_1.html

I did this for my Save & Load system (for NPC nav mesh paths), and I think it works OK, and I hope it might be useful to some other people out there. Or even give people a start on how to do it even better than my version! :)

It's a little bit chopped up to fit in the relevant stuff, without too much other guff. Hopefully the useful bits are all there. :)

These methods are in a UTILS SCRIPT (for serializing Vector3's and things)

 // you can't directly serialize a Vector3, so make a struct to store the relevant info
 [System.Serializable]
 public struct SerializedVector3
 {
     public float x;
     public float y;
     public float z;
 }
 // this method converts a Vector3 into a SerializedVector3
 public static SerializedVector3 SerializeVector3(Vector3 v3)
 {
     SerializedVector3 sv3;
     sv3.x = v3.x;
     sv3.y = v3.y;
     sv3.z = v3.z;
     return sv3;
 }
 // this method converts a SerializedVector3 into a Vector3
 public static Vector3 DeserializeVector3(SerializedVector3 sv3)
 {
     Vector3 v3;
     v3.x = sv3.x;
     v3.y = sv3.y;
     v3.z = sv3.z;
     return v3;
 }


 // this method converts a path into an array of SerializedVector3's based on the path corners array.
 public static SerializedVector3[] SerializeNavPath(NavMeshPath path)
     {
         SerializedVector3[] pathCorners = new SerializedVector3[path.corners.Length];
         for (int i = 0; i < path.corners.Length; i++)
         { pathCorners[i] = Utils.SerializeVector3(path.corners[i]); }
         return pathCorners;
     }
 
 // this method converts the array of SerializedVector3's back into a normal Vector3 array, then turns that into a path using the GetCornersNonAlloc method
     public static NavMeshPath DeserializeNavPath(SerializedVector3[] sPathCorners)
     {
         NavMeshPath path = new NavMeshPath();
         Vector3[] pathCorners = new Vector3[sPathCorners.Length];
 
         for (int i = 0; i < pathCorners.Length; i++)
         { pathCorners[i] = Utils.DeserializeVector3(sPathCorners[i]); }
             
         path.GetCornersNonAlloc(pathCorners);
         return path;
     }
 

These methods are in my NPC script, for saving and loading the nav mesh path

 // this method passes the path into my save & load system
 public NPCData PrepareSaveDataForSelf()
     {
         myNPCData.savedPath = Utils.SerializeNavPath(path);
 
         return myNPCData;
     }
 
 
 // this method takes the saved path, and turns it back into a nav mesh path
 public void LoadEntityData()
     {
         path = Utils.DeserializeNavPath(myNPCData.savedPath);
     }