I think what Dale means is the rat was saturated. In other words, it was wet all the way through the under fur. Generally this only happens when a rat lays in the water for several days.
I had a stretch of creek one time where [b]every]/b] rat I caught, on a 24 hour check, was saturated like that. There was something in that water that penetrated the oil on the rat's fur. The creek was downstream from a golf course, which may have been the problem.
Otherwise, a freshly drowned rat (or beaver) usually has dry under fur. I have no explanation for what happened to that single rat on Dale's line. If it had been fighting in the water, maybe it did get wet that way. But I have caught a lot of rats with fresh bite marks that still had dry under fur. Perhaps this rat was genetically deficient in some manner.
Endeavor to persevere.