I used the pool once during a January trip several years ago. Being in the pool was absolutely fine. In fact, it was great, because there was practically no one else in the pool.
But I was unprepared for what it'd be like after I got out of the pool. That was the cold part. Very very very cold. I shivered like mad all the way back to the room. This was at night and it was, max, 50 degrees outside. I'm sure it wouldn't be so bad during the day--a sunny day.