Read the installation guide that came with the heater.. I will bet the details to your question will be there.
I doubt if the openings are to go in the roof, just within a foot of the top of the space.
If you don't have the book the same info comes with furnaces and sometimes even gas dryers.
How long did the old water heater last.?
If you have a notion not to provide combustion air you will have to bear the consequences of the what ifs, mights and the dreaded coulds.
Since people die from this... the house can never be sold with this condition.. this DIY sin.