Plastic, as a quick rule, has a specific gravity around 1 g/cm^3. This means that it has a density very close to water...some plastics float a little in water while others kinda sink (plastics hover right around floating or sinking). It should be easier to hold down plastic pallets, especially at first when compared to wood. As the wood water logs its density gets closer to the plastic.
A couple cinder blocks per pallet in the stack should be plenty of weight so long as they are in calmer waters. Keep in mind that plastic is much slicker than wood and if the pallets are not level the weight could eventually slide off.
