Pro tip...I know from experience, unfortunately, that 2 cinder blocks will not keep 5 pallets that are bound together submerged. (I am sure that will change though once the wood is water logged. But meanwhile, those "structures" are no longer where I placed them. Harrumph.)
I do not know if 2 blocks will keep one pallet down or not...probably. But give that some thought before it goes under water for good. Also, I would recommend placing your tubes in what will be about 3' to 6' of water when full AND block off one side. Catfish will use them then as spawning cover and if open only on one end, they can successfully fight off hungry bluegills. Many people think that channel catfish cannot, or will not, spawn in a pond. They will...they just need some way to protect all of their fry from roving bluegills.
A one end open tube at the proper depth is just the ticket for that.