Snakes *in* water supply pipes (shower, faucet) highly unlikely.
Snakes crawling along pipes in walls, attic, basement, maybe.
Snakes in drain plumbing (toilet, sink, shower stall), yes. Have seen snakes in toilet bowls.
Snake getting INTO washer VIA any type of plumbing, not likely. Crawling under machine, over tub, to perceived source of moisture, yes.