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.
You can actually email them to ask where you can experience water ski.
Mail requirements are here:
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!