Hedley Whitfield asks in Bulletin No 137 for a practical way to a dry behind......

Obtain a closed-cell foam camping mat as used by back-packers, and cut from it a piece long enough to reach from waist to just above the knees: the leftovers can be made into boat cushions. Cut a leg divide and put a few brass eyelets in the top. Attach with cord to a belt or other suitable garment and stuff it down the INSIDE of the back of your overtrousers. It doesn't do much for the appearance, but it is warm and dry....... and also soft to sit on!

On the subject of warmth, wearing one's buoyancy aid UNDER the waterproof top traps a lot of insulating air and keeps you much warmer than if worn outside.***

Note (2009) This applies only to buoyancy aids. Do NOT wear a gas inflatable Lifejacket inside any kind of jacket. The pressure of the inflation can damage your ribcage and heart.