If statement within FixedUpdate?

I am working with the XR Interaction Toolkit and using the Oculus Plugin. I have a Swimming script using the controller velocity to swim through the "water" but right now, I can only get it to work in FixedUpdate by itself. I want it to only be called when my player enters the water. I have tried to call it in OnCollionEnter/Stay but doesn't seem to work. Can I call an If statement for a collision in FixedUpdate? I cannot figure out how to do that. Here is my code

 using System;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.InputSystem;
 
 [RequireComponent(typeof(Rigidbody))]
 public class Swimmer : MonoBehaviour
 {
     [Header("Values")]
     [SerializeField] float swimForce = 2f;
     [SerializeField] float dragForce = 1f;
     [SerializeField] float minForce;
     [SerializeField] float minTimeBetweenStrokes;
 
     [Header("References")]
     [SerializeField] InputActionReference leftControllerSwimReference;
     [SerializeField] InputActionReference leftControllerVelocity;
     [SerializeField] InputActionReference rightControllerSwimReference;
     [SerializeField] InputActionReference rightControllerVelocity;
     [SerializeField] Transform trackingReference;
 
     Rigidbody _rigidbody;
     float _cooldownTimer;
     
 
     private void Awake()
     {
         _rigidbody = GetComponent<Rigidbody>();
         _rigidbody.constraints = RigidbodyConstraints.FreezeRotation;
 
     }
 
     private void FixedUpdate()
     {
 
             _cooldownTimer += Time.fixedDeltaTime;
             _rigidbody.useGravity = false;
             //add velocity when both controllers pressed
             if (_cooldownTimer > minTimeBetweenStrokes
                 && leftControllerSwimReference.action.IsPressed()
                 && rightControllerSwimReference.action.IsPressed())
             {
                 var leftHandVelocity = leftControllerVelocity.action.ReadValue<Vector3>();
                 var rightHandVelocity = rightControllerVelocity.action.ReadValue<Vector3>();
                 Vector3 localVelocity = leftHandVelocity + rightHandVelocity;
                 localVelocity *= -1;
 
                 //swim forward
                 if (localVelocity.sqrMagnitude > minForce * minForce)
                 {
                     Vector3 worldVelocity = trackingReference.TransformDirection(localVelocity);
                     _rigidbody.AddForce(worldVelocity * swimForce, ForceMode.Acceleration);
                     _cooldownTimer = 0f;
                 }
 
             }
 
             //apply water drag force when player is moving
             if (_rigidbody.velocity.sqrMagnitude > 0.01f)
             {
                 _rigidbody.AddForce(-_rigidbody.velocity * dragForce, ForceMode.Acceleration);
             }
     
        
 
 
     }
 
     private void OnCollisionStay(Collision collision)
     {
           if (collision.gameObject.tag == "Water")
         {
             Debug.Log("Collision Enter");
            
         }
     }
 
 
     private void OnCollisionExit(Collision collision)
     {
         if (collision.gameObject.name == "Water")
         {
             _rigidbody.useGravity = true;
 
         }
     }
 
 
 }