Unknown Shower Scene 1Identified! by various commentors as Thank You (2007). Jang Hyuk!
Unknown Shower Scene 2(most likely) identified! by lili as Bae Soo Bin in Temptation of an Angel (2009)
Unknown Shower Scene 3(most likely) identified! by lili as Park Hae Jin in East of Eden (2008/2009)
Unknown Shower Scene 4
(Probably) identified! by Anonymous and Anonymous (who seems to be enz) as Delightful Girl Choon Hyang (2005). So this would be Uhm Tae Woong.
Unknown Shower Scene 5Identified! by kopytko (with the help of latteholic) as She is Nineteen (2004). So this is Yoon Kye Sang?
Unknown Shower Scene 6
Identified! by kopytko as Love Story in Harvard (2004/5). This is Kim Rae Won, whom I've never seen in anything.