On a few boards there is mention of the 70s release of a 7" vinyl recording of the theme tune; one side in Japanese, the other in English.
This led
here, as indicated by
lindapinda, which credits the English version to
Pete Mac Junior and the Japanese version to a band called
Godiego (actually it is misprinted as Godiegp).
Apparently Godiego also did all of the music for
Monkey, another (similar) Japanese mini-series you may well recall.
Here is a page from their discography (in Japanese!) listing the single. A rough translation seems to indicate the lyrics having been written by bandmember
Thomas Chater Snyder, the drummer and vocalist, who originally hails from the US. The music is credited to
Masaru Sato, a prolific composer of film music, as indicated
here, and it is he who is mentioned in the show's titles.
Finding a direct link to an MP3 of the tune has proved nigh on impossible, however at least one site seems to accept requests for it -
link.
Anyhoo, here is some more info on TWM
http://www.martinday.co.uk/water.htm