Can't make any mobs randomly spawn

Hello! I'm new to MM and I am trying to understand how to get these turtles to spawn naturally in water. So far, I can't get them to spawn period.

I have looked at the random spawners manual options and copied and pasted the "Global Randomspawn Options" example then filled in the blanks and it still doesn't work so please don't just redirect me back to the manual... if I'm doing something wrong that is in the manual, just kindly redirect me to the right area and then explain in more detail...

Can't make any mobs randomly spawn