Nate,
My extruded polystyrene shifted because I didn't use enough tape. My joints were tight. The insulation I used had a thin layer of something similar to plastic wrap ( moisture barrier? Maybe.). Any way that layer pulls away from the insulation if you pull up the tape. I'm thinking that my using only one layer of duct tap (yeah, not Gorilla tape) left only 1 1/2" on either side of the seams. Not a lot of tooth with 15000 gallons of water pressing down not to mention the ocean motion going on when the pool is full of people. Next time, if I do an intex I will triple tape all seams tho, I may like you, forgo the tarp. Like yours, the sun ate the exposed tarp up from around our pool.