Getting them to go into the pond

We have 2 Mallards that previously have a 6in tall kiddie pool. We were digging through our garage and found a 2'x3' water troth(sp?) that has a drain and stop valve already in it and it is a foot deep. We put it in their run ad made huge concrete and brick steps, but they have still not figured it out. The chickens found out in 2 seconds that if they walk up the stairs, there is drinking water. We set it up 2 days ago, and had even put them in so they knew what it was. Went out to check it every day and it is still sparkling clean. We then got sand and built up the sides thinking that they would jump in. Nope, they just stick their heads in and get a drink. How are we supposed to let them know this is their new pool, or will they figure it out on their own?