If you would like to try water ski in Tokyo..
You can sometime see water skiing in Edogawa.
In Japan, you can see lots of people ride water ski in a summer time.
It was actually really popular in late 80's and early 90's but it is not much popular as before.
I found this website and it looks really helpful if you would like to try water skiing.
http://jwsa.jp/modules/info/index.php?content_id=2
You can actually email them to ask where you can experience water ski.
http://jwsa.jp/modules/liaise/
Mail requirements are here:
お名前 *Name
メールアドレス *mail address
タイトル * title
内容 * contents or message
Try to ask them where is the great spot!
yukorose
Japan