A hot tub that is foaming could be caused by the PH balance of the water not being correct so I would first check the PH balance of the water in the hot tub and ensure it's correct.
If not then you need to adjust the PH balance of the water and it should correct the issue with the hot tub water foaming.
Other causes could be soap residue from your ski, fabric softener on your clothes, body lotion, hair products, oily hair and even oily skin.
Your oily skin or oily hair that gets into the hot tub can cause the foamy water in the hot tub so it could be as simple as your natural skin oils causing the foamy water.
Also if your location has hard water then the hard water could be a cause of the foamy water in the hot tub.