If the yacht rolls (ie. while the boat is upside down), will the pvc balls still seal the inflow passages to stop water entry, or will they let water in?
The PVC balls will seal the inflow passages.
Test yourself: 1) fill a kitchen sink with a little water; 2) put in the water a floating ball; 3) open water drain.
Water stream will bring a floating ball to the water drain opening. And, finally, a ball will close the drain.
Here are our test pictures: