Keep spas covered at night and when not in use. Make sure the cover is sturdy enough to bear the weight load of an animal up to fifteen pounds.  This will also prevent any tragic consequences with human toddlers.

Drape a towel or a rough-textured hose securely anchored to the deck with a brick or other heavy object into the pool so the opossum can pull itself out.  The opossum will swim around the perimeter of the pool looking for something to grasp to pull itself out.  It cannot reach up to grasp; the object must be touching the water.  The opossum can get a handhold on most objects except a slick hose with no texture or ribs on it.

Leave a bowl of water nearby so the opossum will not drink from the pool. 

Rescuing an opossum from a pool:  Use the pool net and scoop the opossum and put in a SHADY, secluded spot to dry off (it will sunburn if left in the sun).  You may also use any object the opossum can grasp but be prepared for the opossum to quickly climb up the object and go up as high as it can go- which may be the top of your head.  As soon as it grasps the object you’re using and starts to climb, place it on the ground.  If the opossum just lies there, move it to dappled shade so it can have the warmth of the sun without getting a sunburn.  

Sometimes the opossum has taken too much water into its lungs and it won’t survive.  At this point, there’s no traditional treatment by a veterinarian that wouldn’t cost hundreds of dollars.  There is a homeopathic treatment for fluid retention in the lungs, but we don’t have data on its effectiveness on a drowning victim yet.

The following are commercial products specifically designed for assisting animals having fallen into a pool:

Good for animals up to 1 pound: http://www.froglog.us/ 

Good for animals up to dog size: https://www.skamper-ramp-store.com/regular-skamper-ramp.html