In reading this, I feel like this is kind of a made up problem. If you use a product like that then your pool will still have an overflow drain it will just be an inch or two higher. Once it rains enough the pool level will go up to the top of that drain and you’ll still lose water out the overflow when you get in the pool.
The only time I could see that being useful would be if the pool had an auto top off that was configured to bring the water level up to exactly the same level as the overflow drain. In that case, then the pool would lose water when you went swimming and as soon as you got out the auto top off would replace that water.
However, if your auto top off is configured to bring the water level up to an inch or so below the overflow drain, then there will be an inch or so of play in there. When you get in to swim the water level will rise a little, but not enough to drain water through the overflow. If there has been a lot of rain then the water level might be at the overflow, so water would drain through the overflow when you got in, but when you get out the auto top off won’t kick on because the water level will still be high enough not to trigger it.
This ‘problem’ can be solved by properly configuring your auto top off. And if you don’t have an auto top off, then it’s not a problem at all. When you do manually top off the pool just don’t go all the way up to the overflow drain.
