中國水蛇 Chinese Water Snake, Myrrophis chinensis
微毒 Mildly venomous
Average 40 - 60 cm, can grow up to 80 cm
In Hong Kong, uncommon and restricted in distribution.
蘭花草 翠蘆莉 Ruellia brittoniana 爵床科(草本)
Thank you Mr.HTC for identifying this.
Reference :
http://www.hkwildlife.net/viewthread.php?tid=15459
http://hkcww.org/hkplant/readid.php?id=2202
http://herbarium.gov.hk/subpages.aspx?id=7969