I thought I'd look into 'sopping wet', Hypo, and it was first used in 1897. However, I was rather taken by a quote from twenty years earlier in a farce written by W S Gilbert - he of Gilbert & Sullivan fame - which reads, "Two sopping females have quartered themselves on two dry bachelors."
Hello, I thought, is this some Victorian naughtiness? But, having had a glance at 'Foggerty's Fairy', the farce concerned, it wasn't!