Water is basically slow to render. Some is faster than others. Try out different water mats and find a happy medium. When I find one I like I save it. Also try working with a particular mat. Go into the function editor and eliminate some of the various funcions if you can. Sometimes one will cause the render to be unusually slow. I am not as versed as others so I cannot list some of the slower ones. Maybe someone will provide examples. I sort of know-guess when I see them.