pied d'elephant 'sleeping bag'

[Laminar Flow]

Army surplus used to sell waterproof sleeping bags that had sleeves and a hoodie attached (probably for snipers) that may very well meet your requirements without having to sell the boat to get one.
We had one, but, as we found out, they are pretty useless in the tropics; I don't know what happened to it after.

 

[Mister E]

Check out the waterproof leg covers for use in wheelchairs. Some have a fleece lining and are cheap.