The S3 water rating allows for short immersion in a few feet of water, but a shower is a different experience - water is shot at watch under pressure, and soap is going to be running down your hand onto the watch - soap is slick and likes to get into stuff.
EDIT: Forgot to note that I remember a post from someone on another Samsung phone forum about putting their Samsung S7 into shallow soapy water and the watch failed. Lots of comments there about soap/water combo being a bad idea w/these types of devices.
I'm going to avoid showering w/my watch for above reasons. It may never cause a problem to shower with it, but why risk the hassle when taking my watch off for a few minutes gives my wrist a rest, and I don't really need it or do anything with it while I'm in the shower.