I bought a doogy pool at Petsmart last summer - under $10.00 - Charli loves digging and splashing in the water. Why don't you start with that and see how that goes. Your momma instincts will let you know when it will be tome to introduce the big pool.
I started by putting Webber on the first step of the pool. I got in, and encouraged him to come to me. He did, and then I set him back toward the steps. Over a period of week during the summer I increased the amount of distance he had to go. Eventually, I started throwing a floating device into the pool for him to fetch. By the end of the summer he was jumping off the end of the pool into the water and bringing the floating toy back to me by going up the steps. He was about eight months old when he started these "swimming lessons". He loves the water. At first the dog paddle made me very nervous but once he figured out how to use his back legs, he just took off. It is so much fun to watch and such good exercise. Good luck :-)