When we say High Power, we typically mean more than 160 Newton seconds impulse. I'm going to assume you don't intend to develop that kind of thrust in your first water rocket. I'm guessing that you just mean "a lot of power".
I don't know the answer, but I see from your post history that you have a lot of great questions. I recommend contacting one or more local Rocketry groups.
UFPR appears to have thirty or forty years of association with amateur and model Rocketry; they should be able to point you in a safe and legal direction for your location.
Informações sobre o V Festival Brasileiro de Minifoguetes (2018) estão disponíveis em:
Site do evento:
https://www.foguete.ufpr.br/
Blog do evento:
https://minifoguete.blogspot.com.br/
Facebook do evento:
https://www.facebook.com/groups/minifoguete/ (peça a sua inclusão no grupo)
Grupo do evento no WhatsApp: Festival de Minifoguetes (peça a sua inclusão no grupo)
E-mail:
[email protected]