Most of the pallet type wood breaks down fairly quickly. You make pallets out of cheap lumber not the good stuff.
But, if your lake does not get drawn down and expose your beds to the elements in winter they will last longer. Pallet beds last maybe two seasons tops here because the lake is drawn down. You might get four or five out of them if they stay in water year round.
Hard wood sawmill slabs ripped work good and last. Cedar as well.