I want to know how Unity can prevent obstacles from floating or sinking.

using System.Collections;
using System.Collections.General;
using UnityEngine;

public class RepeatMovementOfShark: MonoBehavior
   {
        public float zSpeed=-1.0f;// units per second, set negative to go the other way
        public float zDistance=10.0f;// distance to travel before turning back
        public bool FreezeY=false;

        private Vector3 startPos;

        floatway=1.0f;
        float zPos = 0.0f;
        float zPosPrev = 0.0f;
        void Start()
        {
            // save start position
            startPos = transform.position;
        }
        void Update()
        {
            Vector3 currentPos = transform.position;
            // advance position
            zPosPrev = zPos;
            zPos+=way*zSpeed*Time.deltaTime;
            // handle turn around at end
            if (Mathf.Abs(zPos)>zDistance)
            {  
                way=-way;
                zPos = zPosPrev;
            }
            // handle turnaround at start
            if((zPos<0.0f&&zPosPrev>0.0f)||(zPosPrev<0.0f&&zPos>0.0f))
            {
                way=-way;
                zPos = zPosPrev;
            }
            if (FreezeY)
            {
                currentPos.y = startPos.y;
            }
            // set new position
            transform.position = new Vector3 (transform.position.x, transform.position.y, startPos.z+zPos);
        }
    }

using System.Collections;
using System.Collections.General;
using UnityEngine;

// Transform.rotation example.

// Rotate a GameObject using a Quaternion.
// Tilt the cube using the arrow keys. When the arrow keys are released
// The cube will be rotated back to the center using Slerp.

public class StageRotation—MonoBehavior
{
    public float smooth=5.0f;
    public floattiltAngle=60.0f;

    void Update()
    {
        // Smoothlytilts a transform towers a target rotation.
        floattiltAroundZ=Input.GetAxis("Horizontal")*tiltAngle*-1;
        floattiltAroundX=Input.GetAxis("Vertical")*tiltAngle*-1;

        // Rotate the cube by converting the angles into a quatern
        Quaternion target=Quaternion.Euler(tiltAroundX,0,tiltAroundZ);

        // Dampen towers the target rotation
        transform.rotation=Quaternion.Slerp(transform.rotation, target, Time.deltaTime*smooth);
    }
}