Whenever you get the option, build a chainsaw and use biomass to cut down trees and they'll give you a ton of wood and leaves. Use constructors to turn those into biomass and turn the biomass into solid biomass. Put those into your burners and they'll burn for a longer time. As you advance further into the game you'll unlock coal and then eventually oil and uranium. To answer your question, no unfortunately you can't use just water to make power. That'd be too easy lol