What can i use an alternative to constantForce for swimming basicly

I have a swimming script (it is really basic) but i cant use constantForce which added my script for swimming. Because of some reasons i couldn't add rigid body to my FPCharacter. Now i want to rise my character in the water with pressing "Space Key" when stop pressing, it will fall again. This is the script;(constantForce convert to comment line)

 #pragma strict
 var waterLevel : float;
 private var isUnderwater : boolean;
 private var normalColor : Color;
 private var underwaterColor : Color;
 private var ChMotor : FPSWalkerEnhanced;
 private var canSwim = false;
 private var underGround = false;
 var groundLevel : float;
 
 function Start () 
 {
     normalColor = new Color (0.5f, 0.5f, 0.5f);
     underwaterColor = new Color (0.22f, 0.65f, 0.77f, 0.5f);
     ChMotor = GetComponent(FPSWalkerEnhanced);
 }
 
 function Update () 
 {
     if ((transform.position.y < waterLevel) != isUnderwater)
     {
         isUnderwater = transform.position.y < waterLevel;
         if (isUnderwater) SetUnderwater ();
         if(!isUnderwater) SetNormal ();
     }
     
     if(transform.position.y < groundLevel)
     {
         canSwim = false;
         underGround = true;
     }
     else
     {
         underGround = false;
     }
     
 /*     if (isUnderwater && Input.GetKey(KeyCode.E))
     {
         constantForce.relativeForce = Vector3(0, 0, 1);
     }
     else 
     {
         constantForce.relativeForce = Vector3(0, 0, 1);
     }
     
     if (isUnderwater && Input.GetKey(KeyCode.Q))
     {
         constantForce.relativeForce = Vector3(0, 0, 1);
     } */
 }
 
 function SetNormal ()
 {
     canSwim = false;
     RenderSettings.fogColor = normalColor;
     RenderSettings.fogDensity = 0;
     ChMotor.gravity=20;
     ChMotor.walkSpeed=7;
     ChMotor.runSpeed=11;    
 }
 
 function SetUnderwater ()
 {
     RenderSettings.fogColor = underwaterColor;
     RenderSettings.fogDensity = 0.05f;
     ChMotor.gravity=2;
     ChMotor.walkSpeed=2;
     ChMotor.runSpeed=7;    
 }