I've never sunk any pallet beds, however I would imagine you would need some concrete blocks tied on. I've put several different kinds of trees in the water (experimenting to see what holds fish best, locations, how easy to handle, etc...) some will sink on their own and others won't. Even the ones that will sink on their own I still like to put weight with them for I'm leary they might move. It's wayyyyyy too much work to loose your bed.
Good luck and let us know how they produce.
Brad
Can You Find, "LIMITVILLE!!!" :D