keep a 2d sprite in an area via color

ok so I'm making a top down game of sorts, I have it so the enemy sprites will move straight towards your base and when they collide it takes health away. how ever only the planes/jets should be able to move over the solid ground (green and tan) however I have a ship sprite that should only be able to move in the water (blue). I looked on the majestic google but found nothing or even a hint at doing this. below is the movement code I'm not sure how to make it so the ship is bound in the water. using UnityEngine; using System.Collections;

 public class EnemyAI : MonoBehaviour {
     private Transform target;
     public int moveSpeed;
     GameObject go;
     
     private Transform myTransform;
     
     // Use this for initialization
     void Awake() {
 
     }
     
     void Start () {
         go = GameObject.FindGameObjectWithTag("Base");
 
 
         myTransform = transform;
         target = go.transform;
     }
     
     // Update is called once per frame
     void Update () {    
                               
         
         //Move Towards Target
         myTransform.position += (target.position - myTransform.position).normalized * moveSpeed * Time.deltaTime;
         if(myTransform.position.x <= go.transform.position.x){
             go.GetComponent <Health> ().DecrementHealth(5);
         Destroy (this.gameObject);
         }
 
         
     }
 }