In both cases the smaller the number, the more buoyant the material. Water has a Specific Gravity of 1 and a value of 62.4 pcf. It is compact, comes in different densities, and has good projection and overall performance. As well as being used as the core it is also used on the deck and rails of most bodyboards. Warm water can sometimes cause polyethylene get too flexible and to develop a rocker and warp over time.
Cold water is better since it tends to stiffen it up somewhat.
Although not as stiff as other cores, polyethylene is still a major player when combined with strengthening agents like mesh or stringers. Nova Chemicals Arcel is a mix of polyethylene and polystyrene. Developed for bodyboards in the mid-80's, this is a light, strong foam used for bodyboard cores. It is a tough, flexible, durable foam that is stronger and more water resistant than polyethylene. Arcel maintains its stiffness even in warmer water but has a tendency to dent.