This month, so you don’t have to, I placed graded cards under water to see how they would fair. I placed 8 graded cards in a pail and held the cards down with rocks and left them there for a week. I cracked the cases for each card and recorded it all.
There were 2 Beckett (BGS), 5 PSA and an SGC. In the end 6 of the 8 cases protected the cards from the water while the other two, a PSA and the SGC, failed. After I finished the video I cracked the final two PSA cases and neither had water damage.