I had a similar experience with a few ducklings and it still leaves a horrible pit in my stomach. They were getting pretty big, nearly three weeks old and I was using a kitty litter pan and they were hopping in and out without any apparent problem, so I removed one brick to give them a little more swimming room (but still had one brick in) it apparently wasn't enough. I was horrified - I had heard the sad stories, but thought I had things covered pretty well. (They were hatched and raised by momma ducks so were supposed to have pretty good oil protection as well.)
I also have one adult runner drake that struggles to get out of the pool sometimes (usually when he's in a hurry because the dog or I am a little too close for comfort), but he seems to have no problem when he's calm - literally "walks" out then. I have a couple of bricks in for him, but I've never seem them use them even when he's struggling to get out. I was concerned that sometimes the water level is too low, but he struggles sometimes even when the pool is full (I think he's just a little less coordinated than the others).
And I do worry about the females when it's heavy duty breeding season.
It does seem our ducks lives are fraught with danger in spite of our best efforts at times.
I have found bricks for both pools and made sure EVERYONE of them knew out to get out before I left!! I have seen where other people put crates in the middle of their pool and always wondered why. I don't know why I didn't ask, so UNLIKE me not to ask
