By cushion I believe you may be referring to the type of foam. If so, you basically have a choice of closed cell or open cell. There are pros and cons to both. Mine are closed cell foam. The are sturdy, and being closed cell they float. Being floating they can be used recreationally while swimming around the boat or tossed to someone who might have gone over the side. On the down side, they tend to be a bit hard and can get uncomfortable over long periods of time.
Open cell foam is more comfortable but will only float until the cells become saturated. Because the open cells are open to moisture, be it sea water
, rain or even dew, mold
and mildew can be a problem after a while. If you go open cell I would recommend that at least the base plates be made of a breathable fabric
and give them frequent airings.
There's a bit more but I think that covers the basics as I understand them.