of closed cell foam.
They do not absorb moisture, and if wet with rain or dew a wipe off with a chamois or Sham-Wow makes them dry in a few minutes

Bottom Siders are similar price and quality , but we found them unhelpful to deal with.

Both these guys put a this decorative strip on the finish. It looks good
If your cushions are symmetrical get stripe both sides so that you can flip if one gets damaged

Our first C cushions were good for 6 years of 5 month/year full time sailing.
Their std thickness of 2" is fine.