Get rid of the Racoons which probably are what is doing that.
Racoons like to swim & drink from pools. And while they snoop around... leave their presents.
Cats are mostly afraid of water ?????

Also cats almost always dig in dirt to do their business then cover it up probably not a cat.

Most animals hate the smell of citrus. Get some lemons,limes,etc, squeeze out and use the juice, then put the rinds in the area where the poop is showing up. It may or may not be cats. Cats do prefer to go where they can dig. It's an instinct to cover their waste,so as to disguise them from their prey.

You might also try asking at the link below. It is a very active forum of pool and spa owners. Surely someone there has had a similar problem.

Good luck.

Here is a link that might be useful: Pools and Spas Forum

We kept cats off our pool cover by setting up a tiny electric fence around it.

The fence wires were supported on dowels, and the dowels were in holes drilled into 6"x6" chunks of scrap lumber. Set the dowels around the pool and string the wire, plug it in.

Typical cat would walk up to it, sniff, get zapped and run off. After a while, they stopped trying.

If there is a certain time the critters are most likely to poop, just have it on then.

sorry, but lol.