I'm no professional trainer, but I would second the choice of using a life jacket initially. Also, be sure to let the dog venture into the water safely and by its own volition. Don't force, pick up, dunk, or drop your dog into the pool.
I'd start by everyone involved with the dog getting into the pool on a good day after a successful and short training session to increase focus and responsiveness.
I'd possibly attach a lead to the dog, just for safety/security, and bring in some of the dog's favorite toys and/or treats. I'd stand by where I'll be instructing the dog to enter the pool and exit, lead in hand, almost in a heel position, and encourage, encourage, encourage!! Stand on the first step, dog beside you, and encourage encourage encourage to step onto the first step! If it takes, congratulate, treat, and praise!! If it doesn't work, do a bunch of play in the pool to get the dog interested, maybe drip some water on the side of the pool by where the dog is standing to make sure the sensation isn't startling or strange.
Repeat as necessary, moving down the steps or ramp, until the dog is in the water and comfortable!