Hmm, does sound lie wet feather you're right. Have you seen her flying about, roosting at a height - is there something else wrong, wing, legs, feet, is the water at flat ground level - can she get in it ok I'm thinking? Is she eating ok? Brown bread squished in milk is a favourite and may help give her some body heat and protein.
Maybe all the rain may help, at least she'll get wet. It's not so cold as yet that staying wet should be a problem overnight. Has she been broody too long and is under the weather?
Ooo it's so annoying when you don't know isn't it! Hope she's ok soon LF