We have a really simple little plastic one and she LOVES it. We put a bunch of water toys in it and she entertains herself for a really long time. My parents also have a bigger blow up pool (8 feet diameter, 2 feet deep) that I go in with her and she loves kicking her legs and pretending to swim in it.
We have a basic one with blow up sides and she basically hates it. She only likes the water if it's really warm and only lasts a few minutes before she tries to stand up using the side and ends up slipping onto her belly. She's getting a water table for her birthday and I think that will be much better.
You can see mine in my BDay AW post. I got it on sale for $20, but I think one without the "stuff" would have been just as good. I wanted one big enough for me to lay in when I was out with him this summer
My sister got her a ridiculous blow up one that has a slide and different sections and blow up toys. You can also set up the hose in it and it sprays water all around it. She loved that part but really didn't go in the pool much. Our neighbor's older kids loved it though! I'd go simple for right now.
Re: Baby Pool
We have a really simple plastic one, too. We've only used it once so far, but she loved just sitting in the water and playing with toys.
https://www.amazon.com/Inflatable-Kids-Shade-Swimming-Pool/dp/B000RPUTBS/ref=pd_sim_sbs_t_1
It's a little elaborate i guess but we like it.