There are loads of sites just try Googling 'watch h20 just add water online' and take your pick
I have found several episodes on youtube just search The Adventures of Sinbad.
usually u can watch them on turbonick.com but they only hav a few
They are currently showing season 2 on channel 10 or can watch the episodes on YouTube.
Netflix, Hulu, Or you can buy all the series on Ebay.
you can watch i think most of them online athttp://www.yidio.com/show/h2o-just-add-water
you can find h2o just add water on DVD at the shops but you have to buy it You can find h2o - Just Add Wather season 1, episode 2 and first 10 episodes from season 3 at: http://www.gtv33.com/2009/08/watch-h2o-just-add-wather-online.HTML
www.youku.com www.megavideo.com www.sidereel.com itunes
Well, you can go on Hulu.com or use the app call Hulu Plus. They have the all the episodes from the 3 seasons.
Turbo Nick nick.com/turbo
You can watch all the episodes on veoh. Some of them are dubbed.
If you have Netflix then just watch it there.
Probably Youtube but just a guess