Chickens normally don't swim, so accidently flying into the pool or pond could mean a drowning. Most chickens will have the sense not to walk into the pool, but sometimes they can be startled and chased into the water. The chances of them getting out of the water are slim.
Unless you have airhead chickens like I do they probably wont swim. I had some lightweight chickens and gave them a bath I actually let them go and they floated on the surface. RIR on the other hand are probably to heavy to float and no one wants to go diveing to get a chicken off the bottom of a pool.
But I think you might have more problems with an inground. Is there anyway you could enclose the pool? I'm thinking for safety,, not only for chickens, but other animals/small children too?