Puppy breath! You are so lucky!

Introducing Chloe to the pool now, when it has just a few inches of water, is perfect timing. At 12 weeks she's too little to do steps. That's learned behaviour and won't happen for another 2-3 weeks. Carry her in and set her down in the shallow part. Bring a toy and tease her with it so that her focus is on the toy and she disregards the strangeness of the water. Six inches will touch her belly and she'll lap at it, might bark at it, and she might feel insecure and try to climb up on you. Hence, the toy. Your job is to help her to associate the water with something fun and pleasant.

Dogs need to learn where the exit is because they tend to go out of a body of water at the same place they went in. From your photos I'm having a bit of a hard time picturing how she'd get out of the pool eventually. For now, guide her to where the steps will be. If there's already a step or the ledge you mentioned that she can reach, then put her toy on it and encourage her to get it. We want Chloe to learn that exiting from the pool is always safe and nothing bad ever happens when she does.

All dogs know how to swim, it's a survival mechanism, but not all dogs like to swim. Some dogs need to learn to like water. If you find that Chloe is afraid, take her out, dry her off, and don't put her back in for a while. There's a small window of time between 10 weeks and 14 weeks which is known as the "fear period" during which a negative experience can have a negative effect on the dog for the rest of its life. You want to avoid this. If she doesn't like the pool, introduce her to a body of water by bathing her in the kitchen sink, a very small area, then graduate to a warm shower in the bathtub. By next season I bet she'll be a water dog.

Have fun with your puppy!