Boat movement, driving and animation

I'm trying to make a player-controllable boat and I'm running into problems with my code.

1) I want to make the boat to accelerate and decelerate steadily instead of simply moving the speed I'm telling it to right away.

2) I want to make the player unable to steer the boat unless it is moving.

3) I've animated an idle animation of my boat floating around, but when the animation is playing, my boat can't move. What can I do to eliminate that?

My current code (C#) is this:

using UnityEngine; using System.Collections;

 public class VehicleScript : MonoBehaviour {
     public float speed=10; 
     public float rotationspeed=50;
 
     // Use this for initialization
     // Update is called once per frame
     void Update () {
 
         // Forward movement
         if(Input.GetKey(KeyCode.I)) 
         transform.Translate (Vector3.left*speed*Time.deltaTime);
 
         // Backward movement
         if(Input.GetKey(KeyCode.K))
             transform.Translate (Vector3.right*speed*Time.deltaTime);
 
         // Left movement
         if(Input.GetKey(KeyCode.J))
             transform.Rotate (Vector3.down*rotationspeed*Time.deltaTime);
 
         // Right movement
         if(Input.GetKey(KeyCode.L))
             transform.Rotate (Vector3.up*rotationspeed*Time.deltaTime);
     
     }
 }

In the current state of my code, when I press the specified keys, the boat simply moves 10 units/sec instantly, and also stops instantly. I'm not really sure how to make the things stated above, so any help would be appreciated. Just to clarify, I don't necessarily need the full code to implement those features, I just want to know what functions to use in order to achieve the desired effects.

Thank you very much.