I too can attest to the fact that drowning children may not make any noise or splash at all. We were at a pool party, my cousin's house. Above-ground pool, probably 4+ feet deep. We had several children in the water, lots of pool toys, floaties, balls, etc. Everybody splashing and screaming, having a blast.
Then someone yelled "Where's Chase?" We all looked, and finally found him at the bottom of the pool. He was sort of balled-up and really just looked like a submerged toy or something. Not moving at all. He was about 7 years old at the time but a pretty good swimmer, had been jumping in and swimming around for quite a while.
To this day we still have no idea what happened, but fortunately he wasn't under long (thanks to all the parents sitting around the pool watching), and he began coughing as soon as he was pulled from the water. He was very confused, awake but not alert, unaware of what was going on. I'm sure only a short time longer and he wouldn't have been so lucky! Everyone was so upset, we all just got out. Party over.
Thanks to the OP for posting, it's always good to have a friendly reminder!