Water snake with a yellow criss-cross on a black background?
The useful relevant facts are:
Present in freshwater
Active in daytime
Yellow patterning
(photos from wikipedia)
I used the site
www.snakesoftaiwan.com/
This gives most likely candidates of:
Rhabdophis tigrinis subspecies formosanus, the Tiger Keelback.
Grows to 0.6 to 1.0 metres long.

more photos (cannot embed due to copyright) at http://m.blog.sina.com.tw/d100vs/article.php?pbgid=30839&amp;entryid=404413
and
Fowlea piscator  the Checkered Keelback or Asiatic Water Snake. growing 1-2 metres long.

more photos at http://m.blog.sina.com.tw/d100vs/article.php?pbgid=30839&amp;entryid=342476