Pallets make great condos. Make sure they aren't made from treated wood or painted.
I've tied brick to them to sink, takes a lot of brick for a 40X48" pallet.
I fished a fence row made of pallets on a large pond. It was great for bass and bream.
During low water we drove post, stood pallets up and tied them together. One condo was 8 pallets in rows 12 to 15" apart. One was a square with 12" open corners, 36X40" pallets spaced in the middle.