I just realized that you are talking about the penetrator holes! I was thinking of the perimeter holes. Nevertheless; if you get to the point that you are compressing the o-ring you should be just fine.
If you are crazy … go to Parker’s site for their O-ring calculator software.
You can select the type of seal that you are dealing with … the o-ring, which should be a buna-n material, and I believe it is a face seal on those penetrators (my memory is offline) and if so you can see how much torque it takes to get “x” amount of compression and run the numbers to see how much pressure that seal will handle.
With a full metal to metal mate, the seal will outlast the metal forming it. I have heard of 100,000+ psi holding pressures with a face seal face.
But back to the soft metal thing, the aluminum is “SOFT” and you will end up screwing up the threads over time. That is why I would be gentle on it. My molds I use at work are made from 6061 and we utilize helicoil inserts wherever we are using attaching hardware to help extend the life on the thread area. As I stated before … I don’t have to tell you the hell that seawater will play on dissimilar metals. Also, be real careful with anti-seize grease if you are thinking of trying that to reduce stress on the threads.
Bottom line, the o-ring doesn’t take a lot for it to create the seal that you need for the depth that the Acrylic can handle.