It'll probably only be about 28" of water height you could get by with 3/4". Just make sure it's supported with probably 2 cross braces and give it 2" of surface area to adhere to around the edges to be safe.
I would not risk 12mm, yea it may hold but it's going to have some serious deflection and could possibly pop the seams. I would spend the extra money for the peace of mind that it's going to be very solid.