Thats the reason for repeated linepicks - the boat can go deeper into the water at any time without overlaps of a fixed sized hole :)
I know that, and I know your way is better. I was just offering a simpler alternative. How about this though; if the sides of the boat had a certain amount of thickness and didn't slope too much, some vertical movement (as well as pitch and roll) might still be possible with a fixed size hole.