If you use sheepskin and loads of lanolin you won't even need a drysuit - or a lifejacket - sheep are always self righting - they float with their legs up in the air!!!
Cotton is the most breathable material around BUT once it gets wet it stays wet. The old rule is "cotton kills". I would rather use a microfibre wicking type material.
There is one amazing cotton based product that is fantastic - Ventile. It was first developed for RAF survival suits during WWII. It is fantastic for mountain use. Much more breathable than modern materials - when it gets wet the pores swell up. It's much nicer to wear and doesn't rustle which is why the SAS uses it. Also much tougher - just chuck your jacket over a barbed wire fence to climb over.
The main problem with Ventile though is that when the water eventually gets through it stays wet - for days!!!